Well, if you are pegging out that fast, you might either be
A) Just starting again. If you are in really bad shape, muscles give out pretty quickly. Keep at it.
B) You are pushing way to hard. Gear down and slow down. You will be able to go farther.
Of course, another thing could be happening. You are not getting out often enough. I have found that the more times one works out per week aerobically, the better. It does not have to be on the bike everytime but getting out just on weekends is not a good thing. If you can do a fast walk or short rides or something twice or three times during the weekdays even if it is for 30-45 minutes, it will help quite a bit.