Started by just walking 3 miles per day, then jogging but mostly walking. One day my kids rode their bikes with me as I went and we got up to 6 miles. The next week I decided to try 7 and I didn't walk at all. Suddenly I wasn't an out of shape man who had to keep stopping to walk over 3 miles, I was a runner.
I ran half marathon distance this Monday morning before heading into work. I'm not going for speed, but my pace averaged to a 10 minute mile which feels decent.
Yet after these weeks of training up, getting up early to do this, it's still a fight every day to try to get up and out there. Not yet a habit even now.