Saddle height is most surly the cause of the thighs getting sore. Somehow my saddle got lowered on my tour bike one of the problems in the 50+ age doing things and forgetting what you did. My saddle was ¾ of an inch low and I had some really bad thigh cramps after about 20 miles to the point of not being able to walk it off. I thought I needed water or something and shook it off forgetting about it till I rode the bike again and around 30 miles cramped again. This time I checked things out and found it low. It actually felt better being low on short rides but for sure was straining the thighs. Seat position front and back will change height a bit also. I like to place the seat about mid position and set height to the heal method and then do the plum bob method putting the front of the knee cap over the center of the pedal with the crank level. Then go back and recheck the height. Keep the saddle level. If anything for myself I error on being back rather than too far forward. Then you have to look at what back angle you want and can your core support it over a long ride. Stem length and quill height are next. For me if I go too aggressive when I get tired I start putting more weight on the arms the shallow arm angle is hard to support weight so I start straight arming in line with the fork and that causes me to move forward.