I've just picked up prototype, and have gotten to the stage where you can roam freely around the city and pick up missions. This includes challenge missions (perhaps they have a different name? I can't recall at the moment) where you have to run around through a set of checkpoints as fast as possible.
Some of these challenges are quite hard, and I might want to return to them after upgrading my speed and such. My question is, do challenges disappear as the story progresses? That is to say, I've done one of the challenges and gotten a silver medal for it. Will I be able to keep attempting the challenge until the end of the game, or will it eventually vanish after running through some of the main plot?