I quite the idea in this article.
Put bluntly, we always over-exaggerate our targets when we first conceive an idea. Our brains are hard-wired to do this apparently.
So take your first goal (I'll run a marthon) and simply chop it in half.
When I was a freshman in college, I wanted to try out for the football team. Given my 5 ‘7″ soft frame, this makes complete sense. You can’t keep a tiger out of the jungle. I decided to become a field-goal kicker. I bought a stand and a football at a sporting goods store. Late at night, I’d sneak into the stadium in Birmingham, Alabama, and practice my kicks. Had I ever kicked a field goal? No. So then why did I think I could walk on as a field-goal kicker for a Division-I college team that occasionally played schools like Auburn? Because I am crazy.