Creepers will mostly either be meatheads, or 40 year old, creepy, divorced bachelor types that are in some kind of bizarre mid-life crisis. They appear to be nice at first, talking to you about the weather, but will make you increasingly more uncomfortable as they begin to violate your personal space, make rude comments / openly hit on you, and make you happy that there's other people around. This is often when you're trying to exercise and use body language to tell them to leave. Add a friend or family member nearby to make the experience more humiliating. Beware, for ridding yourself of a creeper is like removing a cockroach with human intelligence.
Recent testing has shown that creepers are actually more similar to Neanderthals than human beings. This was long speculated by their actions alone, however.
A minute later, you notice him walking over to you. It's like the pit of doom ahead you want to avoid, but you're helpless against the incoming creeper onslaught. You get a closer look at the 40 year old, balding loser, but turn away, hoping desperately that he is targeting someone else. Then, in the creepiest, most annoying voice ever, you hear the dreaded "Hi!"
You turn around, almost snapping your neck because your treadmill is still going fast, and gaze at the perv.
He begins to talk about the weather, climbing on the treadmill next to you and setting it to the lowest speed. He makes a couple corny jokes, to which you fake laugh in order to be polite. Then, it begins.
"So, you look nice today."
(Stares awkwardly at sweaty shirt) "Umm... Thanks?"
"Do you come in here often?" (Or variants of that - never say yes)
He then tells you that he's divorced, and you feel sorry for whoever married this 40 year old idiot who thinks college girls would actually care about him.
You then say, nicely, "I'll see you later," ready to run out the door. You change the time you go to the gym just to avoid the creep.