Mc Hilly, I did a fell ultra in the September on the back of an April P+D marathon. Despite the terrain being ridiculously hilly and not doing much training over the summer, I found it easy. As Wenty said, I didn't worry about paces, I concentrated on getting time on my feet.
LFR - I don't mean to worry you but I applied for London GFA and got my acceptance mag this morning...how come you've got a rejection mag if you're applying via the GFA route? Maybe you should give them a call. (Or maybe I've misunderstood what you're saying).
Madbee - I've not bothered with the general ballot the last couple of years and have just applied via GFA when it opens in June.
I think a combination of what you've suggested and what Medic says will see you right. Slowly build up the endurance (mileage) over the autumn and winter months, tick over on the running front while you focus on the bike race, then fine tune the running over the last 8 weeks. I wouldn't necessarily repeat each week of the 16 week programme.
You probably won't get your best marathon time doing it this way, but you'll have done enough to see you through your first marathon in a decent time.
Incidentally, I achieved a 3.39 marathon time off a 1.47 half time. Granted, it was my second marathon, so I had a bit of experience on my side, but I'd say your 3.45 target is do-able.