I am reading two different symptoms. Cramps vs. Stiffness. One is muscular, the other normally associated with joints. Hips were mentioned, so I'm going with the author is in the final 3rd of his life. Possible arthritis causing inflammation. Also circulation restrictions due to poor support of the body.
For hip joint issues ( arthritis), I take 2 Alieve (Nuproxen) 1 hour prior to flight. Yes the bottle saids 1, take 2. Circulation improvements I use a lumbar support between my back and parachute, which raises my hip slightly for a more even support along with foam throughout the seat pan. Rolled towels under each arm.
I make my cockpit the perfect recliner. 9 hrs, no problem.
I do ankle rolls and arm stretches while cruising to stimulate circulation.
Hydration....yes, very important.
Cheers,

R