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.
How To Halve Your Goals Without Doing LessAt the beginning, when our excitement is through the roof, we think our achievement must be as well. This is why people who have never run 100 yards will tell me they are going to run a marathon. This is understandable. After all, our brains are hardwired to be overconfident about our abilities and chances for success.