Perhaps it's a bit late to comment upon the painful buttock problem, but anyway....
I felt after my first test drive that holding feet off the clutch pedal and keeping light touch on the accelerator was causing strain in my upper legs. Caused, no doubt, by the horizontal body position. So, one of my first jobs was to fit a board on the floor with heel-stops added, one for the left foot and another for the right, positioned separately by trial and error. This enables me to rest my leg and keep foot off pedal when its not required there. So much more relaxed.
