You should be able to, but your cycling will definitely suffer. It has to. If you are committed to running the marathon, then I would recommend scale back the intensity on the bike and use cycling only as recovery/low intensity aerobic, otherwise your potential for injury/illness will go sky high.

Running, I'd recommend, dont do too much too soon. With aerobic fitness in the bank from cycling, your lungs will feel like you should be able to run more than your legs can handle, and you're going to be stupid sore - especially going down steps - for a few weeks. When in doubt, do less. Look through some running forums, those dorks are always injured, and you dont want to end up there. That would jeopardize your marathon as well as your bicycle racing season.

Bonus, running sucks so much that you'll really be motivated to get back on the bike and tear it up come racing season.