Yeah, give it time. It took me about a month to get used to my 15 mile commute. I would get pretty tired after work back then, but after a month it got to be no big deal. The nice thing is that after about 6 months of being a daily commuter you get to a point where you feel extremely comfortable on the bike no matter what distance you're going (as long as it's under 100 miles). Lately, I've been trying to carry as much weight as I can fit in my bag, for "training"