I'd talk to a lawyer, this is very strange. I've done plenty of runs around the fields of college campuses, both my own and others and no one has ever batted an eye. In fact, when I'm driving long distance around the holidays or whatever I sometimes stop in college towns because I know the school will provide me with a place to run. Something about your story seems off.
One thing I've never done is run around the main football stadium, always use a practice field or soccer fields and such. Is that what you did, go into the stadium at a major football university?
You screwed up by lying about the gate being open. You should have said you came in through the hole in the fence and that you are training for track. If you are post college, say you are training to run the penn relays or something. This is generally an accepted activity. Even at a big football school, the worst that should happen is that you are told to leave. I can't believe they'd send a whole squad of cops to kick some guy off a football field on a monday, when it seems one or two cops would suffice.