Why is it that for the first hour or so of every ride I struggle and then seem to find my second wind. Im out twice a week usually for 4-5 hours in general XC type riding but I always struggle on the first few climbs. Quite often I dont even make it up a climb I easily did previously. And then, after an hour or so, it gets easier and I seem to find that second wind. How come? Why do I struggle so much at the start of a ride?