I'm not qualified to judge as I have absolutely zero background in physiology or anything like that, but if I could improve my time on Skyline by 15% next year I would be jumping for joy. I think that as you get more fit, the gains will be harder and harder to come by.
I personally only do about one high intensity run every other week. I can't think of any friend that has done a traditional marathon but many of them do trail runs/ultra marathons. I think that would benefit your mountain fitness more than running on flat terrain. Again, just my amateur opinion. Good luck!
