Well, do intervals and train at altitude. Follow [MENTION=78894]Carbonfiberboy[/MENTION]'s advice. (he's 72 (or maybe 73 by now)) so he understands fighting off age-based decay.

This is the internet age. There must be websites with good advice for aging extreme athletes--what you can do, what you can expect.

I'd never listen to a GP for advice for anything but the most general medical issues. At some point they all say, "You're just getting old." To which one might reply ... "That's just it. I am just getting Old, not decrepit." But to most doctors, preserving life by limiting life is the accepted path---do less and less so you can spend more time doing less and less.