Do I? Or, Don't I?

I know I need to register for the Disney Marathon, but something keeps holding me back... 

FEAR! 

What if I can't do it? 

26.2 miles is a really long way to run. 

What if I can't make myself train?

I'm just too tired to train... another day.

If I sign up, I have to do it. Otherwise I look like a fool to my family and friends.

I'm afraid. 

Anyone else ever been afraid to commit to something, even though you really want to do it? How did you get over your fear?