I was in a similar situation to the OP. I always wanted to exercise consistently, but never really had the motivation to do so. But last year, I had a brief moment of insanity and signed up for an Ironman. Paying $700 for a race gives you lots of motivation. I started as a total coach potato (engineer job sitting on my butt all day) to Ironman finisher in about 11 months by working out about 15 hours a week.