I almost never ride a horse, but took a half-day ride up and back on some rocky trails to some mountain lakes near Bishop, California. This was about a 4 hour ride, not certain of the distance. It was not a pleasant experience! I don' t think it was my keister so much as it was the small of my back that was complaining. It took a few days to stop feeling the aches and pains. A friend who rode weekly had no problem.

I' d recommend finding a riding stable and taking some lessons, get a fair bit of riding time in. That way you can get used to riding and toughen up those muscles.