I am pretty much a new biker. I've had bikes before, but never put a ton of miles on them. I recently borrowed a 'cross bike from a co-worker who had one to spare, and I've done my 8-miles each way commute twice now. I'm finding that the limiting factors so far are the seat ( I can barely sit on the seat since the first ride, my *ss is so sore) and my legs. When I'm going up even a pretty mild hill, my quads (especially around the knee) get this rising, lactic-acid burning feeling that stops me before I even notice my lungs. I remember this being the case with my other bikes more or less, too. Has this happened to anyone else? Does it typically improve pretty quickly with training? What about the seat? Will my butt toughen up, or does this sound like a bad seat fit? BTW, I'm 6'0", 210lbs, 34 y.o.