Hi Mr C,
it's interesting that you are trying yasso 800's in training for your marathon, as I did these as part of my FLM training this year. I trained with a friend at a track and over 12 weeks we built up from 5 yasso's to 10. We ran the 10x800's for two weeks the last being the week before tapering (3 weeks to marathon)then we did 5x800 as we lowered our mileage in the two weeks before the marathon.
I was aiming for a 3.30 marathon, having already done a 3.37 marathon. The yasso's, I too always seemed to run too fast, 3.15-3.19. As I could do them for the 10 I believed this to be ok and I still do. I also always ran 3 miles down to the track and 2 miles, short cut, home.
My long training runs were done at a pace slower than the 8min marathon pace I was aiming for. Apart from I did one 20 miler at marathon pace and several 10-15 mile runs too. As long runs I did something like 2x18 miles, 2x20 miles, 1x21miles and 1x23miles I also did a 15 miler mid week run every week.
Unfortunately I didn't get my desired time having suffered with stomach and breathing problems in the race. But I don't think it had anything to do with the build up as I was getting very good race times, for me, while training for the marathon.
By the way my 1/2m pb is only 1.36, a time I know I can lower, but not anywhere near to your time, so you should be quite capable of sub 3.15. Good luck! My friend who I trained with ran 3.25 finishing with plenty of running still in her legs. Her 1/2m time before the marathon was 1.34 and she knocked 12mins off her previous marathon time.