Honestly, and this may sound shallow. But, go to the Pic section and look at some of the women. Now look at yourself..... My motivation comes from the fact that I want to look good enough that any of these women would find my physical body attractive. I'm married with kids and happily so. I figure that if a woman I don't know find my appearce attractive then my wife will also. Believe me, girlfriend's and Wives, are the worst at being honest. But strangers are not. If you feel a stranger would find your appearance appealing, you will gain alot of self worth and self confidence. I want to look the best for my family. Looking good usually means that your also healthly and isn't that what all this is about, Looking good, feeling good, and feeling good about yourself?
If this doesn't work, GoPro will just kick you in the ass and believe me that is more than enough motivation. Accountability is a great motivator.
Good Luck