conventional wisdom is that being in the back seat is the cause of quad fatigue, but it's not the only possibility.
The worst thigh burn I ever had was from skiing on som old slalom skis with the bindings too far back and getting really far forward to compensate and pressure the tips. The day had a lot of mogul-like slush piles, and I spent a few hours hammering the ski tips into the slush piles to crank out tight slalom turns. Result was that while the piles helped load the tips and crank out the tight turn they also exerted considerable drag. The constant force of the quads trying to advance the feet forward and taking the extreme loads with the shins far forward did wear me down (age happens
My prescription is to spend a couple of hours three times a week doing very intense hard slalom turns on a short radius ski. That ought to fix you up.