Just start!

You aren't going to be the fastest, strongest, or most fit from the start. You'll probably never be the fastest, or the strongest, or the most fit, there's always going to be someone more tough than you. Good news is, they aren't your competition, you are! Strive to be the best you that you can be. Better than you were yesterday. All you have to do is START. And you'll be lapping everyone who is still sitting on the couch.