I have a NordicTrack EXP 1000 X. Just bought it froma guy on craigslist, and it was working fine at his house. Messed with the belt tensioner bolts a bit when I brought it home, and now when I walk/run on it, after about 1 minute or so, it slows down to a crawl and almost stops. Am I too fat for it or did I screw something up?? Thanks in advance for any help you can provide!

It sounds like the belt is bogging down, this condition is when the treadmill operates normally without a person on the belt and then slows down when someone steps on the belt or when the treadmill operates normally for a given period of time with someone on the belt then abruptly begins to slow down.
The only certain way to test for a worn walking belt is to take a DC amp draw . Trying to look at the belt or a feel test is highly unreliable. Better tests, if you lack a DC ammeter (they are expensive for a good one), are a coast test or an incline test.
The coast test is to get on the treadmill as the lowest incline setting and walk on the treadmill at 3 MPH. Pull the safety key and it should take you 2-3 full steps to stop fewer steps indicate high friction.
The incline test is to put the treadmill at max incline and walk on it at 3 MPH. If the treadmill operates normally at max incline but bogs down at minimum incline, replace the walking belt. Gravity takes over for the drive system eliminating much of the friction problem. On some heavily worn walking belts, this test will not eliminate the problem.