Posted: 29/09/2014 at 19:41
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.