Do or not do. There is no try. Still, it's true. If what you're trying is worthwhile, put yourself into it completely or you'll just increase your chances of failure. If you are I'll bet this is the reason I beat you.
Most opportunities present themselves only once.
Especially the really good ones. Many times when road racing bikes I spent an entire race behind someone I should have passed because I hesitated when I had the opportunity. If you want to run your first marathon, or achieve any other personal goal that is important to you, you should go for it when you have the opportunity, even when you're afraid of the consequences should it go wrong. When I raced bikes I got to learn this lesson several times a weekend until I found a way of dealing with the fear and taking advantage of opportunities when they appeared. There are some goals for which opportunities might appear only once in a lifetime.