I bike commute in the spring, summer and fall and it's VERY good cross-training. (I even bike commuted while I was training for a half marathon...and the day after the half marathon!) However, you have to remember that it uses different muscles than running, so it's not surprising that your quads were torched after your first 15 mile ride.
When I first started commuting, I backed way down on the running until my body got used to it and I built up some strength. I'd suggest you do the same thing with the biking - start really small and work your way up. Like Judi said, it will only get easier!
"Try not. Do. Or do not. There is no try." - Yoda
Half Marathon - 2:25:24 (Gary Bjorklund HM 6/18/11)
5k - 30:37 The Big Chill (2/27/11)