Phil,
I am a 40 year old, 5'10" man, and I used running as part of a weight loss plan. When I ran my first 10k race I weighed about 13.5 stone, and finished in a little under 44 minutes. Just over a year later, with my weight down to 10.5 stone, my 10k time was down to just under 35 minutes. Now, weight loss on its own didn't account for all of the improvement - I was training pretty seriously as well - but it certainly helped. The other point to bear in mind is that weight loss improves your ability to train hard and consistently, since it reduces the impact on joints, and therefore the risk of injury. The optimum running weight for me, by the way, is about 2lbs for every inch of height, plus 10 lbs. This calculation gives a BMI towards the bottom end of the healthy range of 20 - 25.
Cheers
Daniel