"Pippa has been stepping up her workouts getting ready for her wedding," a source close to the bride-to-be tells ET. "She's been working out about four or five times a week."
PHOTOS: Pippa Middleton's style through the years
Pippa Middleton's style
Pippa Middleton's style
Pippa looked breezy in a lilac wrap dress while out in London.
Pippa attended a book launch in a pretty black dress with lce sleeves.
Pippa attended Sotheby's Summer Party in a bright summery dress.
Pippa wore this black and white belted dress to a book launch party.
Middleton looked beautiful in a beaded gown at a charity gala in London.
Pippa wore an Issa dress to the label's show at London Fashion Week.
The socialite attended the Day-Glo Midnight Roller Disco alongside her sister, Kate, wearing a sequined mini.
Pippa attended the End of Summer Ball in a flowing v-neck gown.
Pippa chose this bright blue frock for the 10th Anniversary celebration of Claridge's Bar.
Pippa wore this stunning white wrap dress to a polo match with white wedges.
Kate's younger sister wore a tiered yellow dress to Tatler's 300th Anniversary Party.
Pippa added a pop of red to her simple black dress with a bold belt.
Pippa attended a fundraiser in a metallic-embellished dress.
Pippa wore a black tiered jacket and bright pink fascinator to a friend's wedding.
The day before Kate's wedding, the family arrived together. Kate wore a patterned Issa dress while her sister was pretty in pink.
Pippa was dressed in an Alexander McQueen gown for her sister Kate Middleton's wedding to Prince William.
The morning after the Royal Wedding, the Middleton clan left their hotel. Pippa was spotted in bright white pants and a blue Zara blazer.
Pippa accessorized her all-denim look with a neutral bag.
The Duchess' younger sister stepped out carrying her favorite Modalu bag, wearing jeans and a bright pink top.
Pippa chose a blush colored Maje top and carried her Modalu bag while out in London.
Pippa wore a crocheted white dress and a jean jacket to the AEGON Championships.
Out for a shopping trip, Pippa chose a pretty blue and white wrap dress.
Middleton attended a Wimbledon match in a navy version of her crocheted sweater dress.
Pippa met up with friends wearing this casual denim dress by Whistles.
Pippa wore this pretty two-tone Zara dress while out in London.
Pippa dressed up a plaid Maje top with red flats and a Prada messenger bag.
Middleton wore Temperley London's classic fan dress during London Fashion Week.
Pippa stunned in London wearing a tiered silk dress by French Connection with a tan Whistles jacket.
Pippa got coffee with friends in a white Issa dress, adding a pop of color with her Prada bag and matching red flats.
Pippa glowed at the Boodles Boxing Ball in a red gown by Temperley.
The Duchess' younger sister looked pretty in plaid by Zara and black skinny jeans.
Pippa looked chic in a longsleeved LBD, an Alice by Temperley animal-print scarf and a Prada bag.
Pippa attended a charity event in a stunning floral gown by Project D.
Pippa wore a cobalt blue turtleneck dress with a chic black blazer and lace-up booties.
The socialite bundled up in a funnel-neck grey Fay coat and black boots.
We're slightly obsessed with Pippa's blue Sara Berman coat with suede trim which she wore with a black dress and booties.
While out in London, Pippa wore a chic army green Fay coat with black skinny pants and a bag by Loewe.
The royal-in-law wore a snowy white coat with black boots and gloves.
Pippa went for a stroll in bright pink jeans by SuperDry, a plaid Maje top and Zara blazer.
Pippa wore a navy Orla Kiely jacket as she and brother James Middleton smile for the camera.
Pippa attended a Wimbledon match in a navy dress and macramé jacket by Orla Kiely.
Pippa wore this and white Alice Temperley dress to the premiere of Shadow Dancer, completing the look with strappy heels.
Pippa wore a bright colorblocked Paper London dress to the U.S. Open.
Our favorite royal-in-law went out shopping in NYC carrying a Kate Spade bag, and wearing a casual but bright outfit.
Pippa attended the U.S. Open in a bright yellow Phase Eight frock.
Pippa wore Stella McCartney to promote her book Celebrate: A Year of Festivities for Families and Friends.
Pippa hosted a Halloween Party for children in London while promoting her book, wearing a brocade Markus Lupfer dress.
She attended a party for W&W Jewellery, a British jewelry company in a metallic skirt and neutral top.
Pippa traveled in style weraing a pretty eggplant coat.
Pippa promoted her book Celebrate in Amsterdam wearing a bright purple peplum dress by Beulah London.
Pippa looked cozy at the Cheltenham Festival in a Katherine Hooker mustard yellow coat and the same Aquatalia boots her sister Kate loves.
As the new ambassador for the Mary Hare School for the Deaf, Pippa opened the "Murray House" in head-to-toe Tory Burch.
Kate's sister stepped out in London wearing a top and skirt by Sandro, jacket by Alice by Temperley, Alice + Olivia heels, and the Catherine Street Pippa bag by Kate Spade New York.
Pippa attended a party in a pink floral dress by Tabitha Webb.
The middle-Middleton was out and about in silk ikat pants by Alice by Temperley.
Pippa out and about with boyfriend Nico in London, wearing a chic black dress.
Pippa wore a pretty green sweater dress and matching cardigan at the AEGON Championships with her mother Carole.
Attending a friend's wedding, Pippa looked lovely in a black and white peplum Tabitha Webb dress and Olivia Roat Millinery fascinator.
Pippa looked chic in a powder blue blazer over a matching dress at Wimbledon.
Pippa attended Spectator Magazine's summer party in a pretty white lace dress.
Pippa went to another game at Wimbledon with boyfriend Nico, wearing a striped coral dress by Temperley London.
Leaving Andy Murray's match, Pippa looked vibrant in the coral dress.
Pippa attended the Queen's Coronation ceremony in a floral Tabitha Webb dress with a lace overlay, and carried a clutch by Lamb 1887.
Pippa attends the wedding of James Meade and Lady Laura Marsham.
Pippa and Nico Jackson pose at the Boodles Boxing Ball 2013 on September 21, 2013 at the Grosvenor House in London,England.
Pippa attends a donors dinner hosted by Michael Bloomberg & Graydon Carter to celebrate the launch of the new Serpentine Sackler Gallery.
Pippa and James Middleton arrive at Chapel Royal in St James's Palace in central London to attend the christening of Prince George of Cambridge.
Pippa and boyfriend Nico Jackson attend the Sugarplum Ball.
Pippa attends a memorial service for Sir David Frost at Westminster Abbey.
Pippa attends day two of the Aegon Championships at Queens Club.
Pippa Middleton seen arriving at Wimbledon.
Pippa attends the GQ Men of the Year awards.
LONDON, ENGLAND - SEPTEMBER 23: Pippa Middleton attends the launch of designer and entrepreneur Tabitha Webb's first retail store 'Tabitha Webb' on Elizabeth St, Belgravia on September 23, 2014 in London, England. (Photo by David M. Benett/Getty Images for Tabitha Webb)
LONDON, ENGLAND - FEBRUARY 10: Pippa Middleton attends the inaugural Roll Out The Red Ball in aid of the British Heart Foundation at The Park Lane Hotel on February 10, 2015 in London, England. (Photo by David M. Benett/Getty Images)
LONDON, ENGLAND - MARCH 18: Pippa Middleton leaves The Cumberland Hotel after attending a fundraising ball for Disability SnowSport UK on March 18, 2015 in London, England. (Photo by Keith Hewitt/GC Images)
LONDON, UNITED KINGDOM - JULY 9: Pippa Middleton seen arriving at Wimbledon on July 9, 2015 in London, England. Photo by Neil Mockford/Alex Huckle/GC Images)
LONDON, ENGLAND - FEBRUARY 11: Pippa Middleton attends the British Heart Foundation: Roll Out The Red Ball at The Savoy Hotel on February 11, 2016 in London, England. (Photo by Danny Martindale/WireImage)
LONDON, ENGLAND - JULY 18: Pippa Middleton and Princess Eugenie attend The Frost family final Summer Party to raise money for the Miles Frost Fund in partnership with the British Heart Foundation on July 18, 2016 in London, England. (Photo by David M. Benett/Dave Benett/Getty Images for British Heart Foundation)
ENGLEFIELD GREEN, ENGLAND - MAY 20: Pippa Middleton (R) is escorted by her father Michael Middleton as she arrives for her wedding to James Matthews at St Mark's Church on May 20, 2017 in Englefield Green, England. (Photo by Justin Tallis - WPA Pool)
SYDNEY, AUSTRALIA - JUNE 01: (EXCLUSIVE COVERAGE) Pippa Middleton is seen at Sydney airport on June 1, 2017 in Sydney, Australia. (Photo by Matrix/GC Images)
LONDON, ENGLAND - JULY 05: Pippa Middleton seen arriving at Day 3 of Wimbledon 2017 on July 5, 2017 in London, England. (Photo by Neil Mockford/GC Images)
LONDON, ENGLAND - JULY 16: Pippa Middleton seen on day thirteen at The Championships at Wimbledon on July 16, 2017 in London, England. (Photo by HGL/GC Images)
January 7, 2018 in King's Lynn, England. (Photo by Max Mumby/Indigo/Getty Images)
WINDSOR, ENGLAND - MAY 19: Pippa Middleton and James Matthews arrive at the wedding of Prince Harry to Ms Meghan Markle at St George's Chapel, Windsor Castle on May 19, 2018 in Windsor, England. Prince Henry Charles Albert David of Wales marries Ms. Meghan Markle in a service at St George's Chapel inside the grounds of Windsor Castle. Among the guests were 2200 members of the public, the royal family and Ms. Markle's Mother Doria Ragland. (Photo by Chris Jackson/Getty Images)
PARIS, FRANCE - MAY 27: Pippa Middleton and James Matthews are seen in Roland Garros on May 27, 2018 in Paris, France. (Photo by Stephane Cardinale - Corbis/Corbis via Getty Images)
LONDON, ENGLAND - JULY 05: Pippa Middleton seen arriving at Wimbledon Day 4 on July 5, 2018 in London, England. (Photo by Neil Mockford/GC Images)
LONDON, ENGLAND - JULY 13: Pippa Middleton seen arriving at Wimbledon for Men's Semi Final Day on July 12, 2018 in London, England. (Photo by Neil Mockford/GC Images)
LONDON, ENGLAND - JULY 11: Pippa Middleton and James Middleton seen on day nine of The Championships at Wimbledon, London on July 11, 2018 in London, England. (Photo by HGL/GC Images)
WINDSOR, UNITED KINGDOM - OCTOBER 12: (EMBARGOED FOR PUBLICATION IN UK NEWSPAPERS UNTIL 24 HOURS AFTER CREATE DATE AND TIME) Pippa Middleton attends the wedding of Princess Eugenie of York and Jack Brooksbank at St George's Chapel on October 12, 2018 in Windsor, England. (Photo by Max Mumby/Indigo/Getty Images)
LONDON, ENGLAND - DECEMBER 04: James Matthews and Pippa Middleton seen attending The Henry van Straubenzee Memorial Fund's Christmas Carol Service at St Luke's Church on December 04, 2018 in London, England. (Photo by Ricky Vigil/GC Images)
LONDON, ENGLAND - FEBRUARY 27: Pippa Middleton attends The Beating Hearts Ball in support of The British Heart Foundation at The Guildhall on February 27, 2019 in London, England. (Photo by David M. Benett/Dave Benett/Getty Images )
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"She's been working with a personal trainer at a gym near her home in Chelsea and also has been doing Pilates," the source continues. "She looks and feels great -- she's never looked better."
The brunette beauty was all smiles when she was spotted leaving the gym in London on Wednesday, looking fit and fabulous. Middleton showed off her toned legs in a pair of black-and-white athletic shorts, which she paired with a gray pullover and blue New Balance sneakers.
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"Pippa has always been active and athletic, but she's very lean and muscular right now," the source adds. "She's been detoxing with a healthy diet and also has been getting regular facials. She's also been visiting her favorite salon, Richard Ward, for hair treatments."
ET previously reported that Prince Harry's girlfriend, actress Meghan Markle, has requested a week off from shooting Suits to join him for a portion of the couple's wedding. She is unable to attend the ceremony at St. Mark's church in Engelfield, England, however, as protocol prohibits unmarried and un-engaged partners to be present at the nuptials.