You probably ran faster than you are used to, is all. It really takes it out of you.
Go out on the road at your own pace, and you'll feel much better.
Also, did the run start up hill? Do you put the treadmill on an incline? If you don't you are going to have difficulty outside, where there is wind, and gradient, to make it much harder.
I can run waaaaaay faster on a treadmill than I can outside (although this is the opposite to what many people find). If you're the same, you'll just have to go a bit slower outside.
Edited: 19/08/2008 at 12:41