Rich: I second wot the others said. No need to force 50 mile weeks on your body just yet. Do what you can accommodate. If you are not used to such mileage you may end up injured. But once you get a good base going you may want to try a bit more the beauty about HADD is that it's gentle on the body, rather than speedwork which tends to break a lot of people, especially as we get older.
I'm still trying to do most runs easy with the odd 80% run but as I'm part of the relay team from work at the Prague marathon I've done a bit of faster stuff too, namely some intervals not exceeding 90% of max. and tempo runs around 87/88% of max. Things are slowly improving
Today I ran my longest run since Boston last year. 19 miles @8:58 average pace, HR 71%. Felt strong throughout. Cautiously optimistic about an autumn marathon but first it's Prague and then a half in June where I hope to come close to my old PB (1:34:00).