Pathfinder, any good instructor is going to work with you, let your instructors know up front your capabilities, and the reverse.
I would also recommend for anyone taking a top notch course, to start preparing for the course. When Thunder Ranch was here in Texas, they held courses all year, July and August can beat the best of us, if we have'nt done some preparation for the heat and exposure. I don't know how bad your condition is, but try getting some high grade knee pads and working with them slowly with something to hold onto while practicing. Unlike a handgun, a rifle or carbine can be used to help yourself up if done properly and safely. I am 49 years old, and have been in the automotive business most of my life, getting on my knees kills me, but I've gone to wearing 511 pants with the knee pad inserts, and they really help. That 1/4" of foam does'nt look like much, but it works.