If you want motivation for losing weight, it is generally said that every pound you lose is worth 2 secs/mile over the marathon distance. The bad news is that for every year you are over 50 you lose 4 secs/year.
As I only did my first marathon at 50 I have had to lose 2lbs/year just to stay still.
When I started running I was a hefty 12st10lbs (5'7" tall). When I set my current pb I was 9st7lbs