Hi i'll do my best to answer everything. Last year this was harder than the marathon though but luckily was only a hour or so!
As further background, this is my 35th successive year of preparing for a marathon and the last 5 years i've been reasonably consistent at London (with 2:46, 2:43, 2:46, 2:43 (my quickest London in last 20 years), 2:47. Over the years I've estimated I've done around 2000 races - I have over 1200 logged on the power of ten website.
Ferret.
I would keep the sessions in but just rein the speed in a little and take them slightly easier. This way you should be able to improve your aerobic capacity and be able to accelerate the speeds gradually rather than do lots of slow running and then start from scratch with the faster runs. As long as you are doing the faster and tempo runs quicker than your long runs then it will be doing you good. They don't have to be at your pre break pace.
Jokerman
For me the most important session of the week is probably my weekly race, though I wouldn't encourage everyone to race as much as I do!
As regards to normal sessions, I think the most session is a hard speed session ie intervals of 400m, 800, mile etc. This I think leads to the biggest improvements in your speed endurance and causes a knock on effect at all distances from 5k to marathon. In my sessions I do like to get a range of paces though and even in the course of a session of mile reps, some part would be nearer marathon or half marathon pace, and some even at 10k pace, even if the majority is at 10k pace.
A marathon runner though does need to do some of his training at marathon pace and so you can't just relly on the fast speed sessions