Hey Mitts, me again!
As a veteran of, ahem, ONE FLM, I can sympathise with you. Last year I came into the training on the back of 2 halfs, best time 1:44. I wanted <4 hours, secretly aiming for 3:40. I managed 3:59 and was fairly happy with that. I followed the runner's world intermediate running plan, but had a few criticisms!
I felt that there was a fair amount of 'pulp mileage' mid-week and no rest. I 'tweaked' it a little to fit my lifestyle and ended up with 35-45 miles a week. On the day, I ran a safe-4 rather than risk it all for 3:45. As it turned out, it was a conservative strategy and I could have run faster (they all say that!).
This year, I have a best half of 1:39 and a bit more confidence at the higher mileages. I'm doing the Bury 20 (as you know) and will do 3 other 20+ runs. At the moment I am confident of holding <8min/mile based on a schedule of 40-50 miles a week.
I guess the best advice is to arrange your plan to your lifestyle and listen to your body. You'll know if it's too much. If you can get your Sunday runs up to 20 without needing 3 days off work afterwards, you'll have confidence for April 17th. Based on my HUGE (er, 2 years) experience, confidence is worth 20 minutes off your time!
Good Luck Mitts
Graham