First of all, yes, I would say your progress is good. It is certainly nothing to worry about. Many people can't maintain 7.35min/miles.
I make it that in old money, you have gone from 17stones 2lb to 14 stones 6lbs. That's going to have done you the world of good, I'm sure. Your resting pulse look svery healthy.
I don't know an awful lot about heart-rate training, but you seem interested in what your heart is doing, so maybe following a heart-rate-based schedule would be a way you may like to train? It's also a relatively easily-measurable way to train, too. Don't think that you have to be doing 40+ miles per week to be doing yourself plenty of good, but you would have to increase your mileage for a marathon. It is only necessary to do this for a few months before the race, though.
A marathon certainly seems well within your potential.