like Knotted said, look at tweeking your fit.

I've found that one of the biggest obstacles to upping mileage is getting the rest of you body used to being in the cyclist position for that period of time. I think it takes getting used to. I find that rather than fatigue the things that make me the most uncomfortable as mileage increases are things like my bum hurts, arms hurt, hands hurt. And usually as I log more miles and lomre longer miles those things work themselves out. (the body gets used to them) or you find better gloves, or better shorts.

It also really helps to change positions as much as possible. Moving your the position of your hands, stretching our your neck, round out your back.

Have you ever done a spin class? in spin class we were taught to lead the class in upper body stretched at the beginning of the class, mid class and end of the class. easy little stretched to loosen up and not get stiff on the bike. I do these on my road bike if I feel that I'm getting stiff. Good to do on breaks too.