Mark,
To echo a notion already expressed here: Running is not the best workout for all of us.
I joke with hiking partners, but it's true: I'm the descendent of women built to pull plows in Bavaria, not built to move fast.
Running for fitness is simply not an option for me. While I still played soccer (I quit shortly after turning 30, which I consider a prudent age to lay off this wonderful but joint-thrashing sport) I tried to run to build up my speed and endurance. All I ended up doing was suffering knee pain and developing achilles tendonitis. I soon spent more time in physical therapy than I did running or playing soccer. I had a ortho look me in the eye and say: If you want to remain active into your 40s and beyond, you'll stop running.
So, I'm not one to discourage anyone from taking their fitness to the next level. I think it's great that you've got the drive and discipline to stick to a running regimine. I just want you to consider that maybe running is not the right fitness activity for you.
For comparison/ideas, my fitness regimine is based on heading to the mountains each weekend, briskly walking our dogs 1.5 miles several days a week, yoga and core strengthening. That's it. I'm 35 years old, I can eat whatever and as much as I please and not gain an ounce, and I'd be surprised if anyone on this site who's summitted peaks with me would say I'm slow or out of shape. No running, no suffering, no joint pain.
Take good care of your body, keep up the drive & ambition and MOST IMPORTANTLY: Have fun!
Cheers!
Sarah