I can't offer an answer, but I'd also be interested to hear people's thoughts on this...
My situation is that I have an October marathon planned, so I'm aiming to peak for that, but then possibly build to a spring marathon (with sub 2:55 as the target) IF I find that I like marathon training in general.
My plan for the summer, is 4 weeks of easy running, then an adapted version of Daniels 2Q plan (56-70) for 14 weeks. Mileage will average low-60's with peaks of 70 (3 weeks). I was planning on more easy running to begin with, maybe 8 weeks, but I've decided to go with just four as I've got a half marathon mid way through the marathon plan, which is arguably just as important - don't want to go in to that one off of virtually only easy mileage.
Like you I have been averaging around 50mpw over the spring, although this has been tapered off over the past month and a half due to races. Also like you, I feel like endurance is my weak point at the moment, as my 10k is slightly better than my half (didn't do enough long runs) - marathon training should sort that out though I hope!
October will be my first marathon, but 3:00 is pretty much out of the question regardless, as the course is very hilly in the second half.
Will probably build up to average 70-80 over the winter, if I decide to do another in the spring, but I'm in two minds at the moment as I've really enjoyed the recent 5k/10k training... Go with what I enjoy best I suppose!