The only time I stop during a century is to refill my water bottles. I don't like to stop when taking in a gel pack. Just pace yourself, you certainly don't want to burn yourself out early. I usually don't feel like I'm getting worn some until around the 75-80 mile mark, that's when some lactic acid begins to set in for me. Be sure to take in an even balance of sodium and potassium, because you're going to be sweating a lot, and you need to put that back into your body. And drink drink drink!