I have a buddy that is a runner, and over 2 years ago when I quit smoking, he signed me up for a trail run. It was intimidating, and hard, but I got so much out of it. After that, I signed up for a themed obstacle mudrun with my lady and some friends. We had a blast. Then I found out about Tough Mudder, a 12 mile obstacle mudrun. I did that last Feb. and am signed up to do another next month. I also do 5k's.
The act of signing up for events and putting them on the calendar motivate me so much to get ready for them, its very effective for me. I get that mudruns aren't for everyone, or running for that matter. I'm 265lbs and hate running, but I love the way I feel after a good run. So whether its a bike race, 5/10k, mudrun, trail race, just sign up for something. You don't have to compete against anyone but yourself. And enjoy the experience.