Pippa, 30, and her brother James Middleton, 27, have both registered for the Race Across America, starting on Saturday.
The Middleton siblings will be a part of an eight-person team making the 3,000-mile journey from Oceanside, California to Annapolis in Maryland.
Pippa and James Middleton
They are taking on the physical challenge in memory of their team-mate James Matthews' late brother Michael, who died in 1999 during a descent from Everest.
Pippa Middleton nails summer chic in Tory Burch dress
It is not the first time that Pippa has tested her endurance; she completed a 56-mile cross-country skiing race in Sweden in March 2012, and the previous year took part in the Highland Cross Challenge, running and cycling 50 miles coast-to-coast.
Sporty Pippa ran the Blenheim Triathlon in 2011
Her other interests include sailing and swimming; and she is an avid tennis fan, a regular spectator at tournaments including Wimbledon and The French Open.