Pippa Middleton is ready to say "I do!"

The 33-year-old sister of Kate Middleton is gearing up for her upcoming May 20 nuptials to James Matthews, and ET has learned exactly how she's prepping for her big day.

WATCH: Pippa Middleton Is Engaged to James Matthews!

"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."

"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."

Great, indeed!

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.

"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.

WATCH: Meghan Markle to Attend Pippa Middleton's Wedding With Prince Harry

We also know that the Duke and Duchess of Cambridge's children will play a big part in the wedding. Prince George, 3, will be a page boy, and Princess Charlotte, 1, will serve as a bridesmaid.

Hear more on Middleton's highly anticipated nuptials in the video below!