I'm in the same boat, although I have the advantage of being 20 lbs over my ideal race weight. I plan on running it to get my weight down for cycling and for Baker to Vegas. My running 'season' will end March 15th.

My training is loosely set like this:
Monday off
Tuesday run 5m
Wed run 5m
Thur run off
Fri cycle 40m, run 3m
Sat cycle 20m (easy)
Sun cycle 40m, run 3m

I know some people train alot harder than this but right now the 2 a days put me on the edge of injury. Once my body is stronger I plan on alternating weeks to have a 60+ mile ride or a 10+ mile run.

Anywhoo, personally I think if you are in race shape it may hurt some to train for a marathon, its not really something you can quasi commit yourself to which means less time on the bike.