We like to stay in Ranchos 6A. There is a short (3 rows, I think) parking lot between bus stop #3 and the entrance to the building, and the elevator is right there when you walk around the corner. With the exception of DTD buses, the buses stop at bus stop 2,3,4, then 1. We hardly ever found the buses crowded when coming to our stop, but it usually packs out at #1. The walk from Ranchos is less than 5 minutes to the main pool at a leisurely pace, and less than 10 minutes to El Centro. Because the grounds are so beautiful, this is a very nice walk - especially at night when the lights around the lake are lit. However, if walking is an issue, there is an easy solution: As all buses (except DTD) stop at El Centro last, you can hop any bus at your stop, and get back off at El Centro. To get back to your room, wait for the DTD bus, as it stops at 1 first, then goes around the loop to the other stops. The internal shuttle is nice, but only available during the day. We have always found the buses the quickest way to get to the front of the resort.
BTW... don't let the size of the resort scare you. CSR has less rooms than the values in a much larger area. This gives the resort a very peaceful, quiet feel. On several occasions, we walked around the lake and only saw a handful of people!