Replying to Late season fatigue...

It seems like the last few times ive gone up, my legs get tired faster than they did earlier in the season. Its really my first season of going up a lot, probably 40-50 days this season. I was wondering if you guys have had the same thing, like maybe the legs get worn out of the course of a season or something. I went up yesterday and by 4pm my legs were toast, I just couldn't do shit. I am gonna train hard this summer to ensure this isn't as much of a factor next season, but damn it sucks not being able to tear it up all day like I did back in December and January.