Maintainance of bike power and fitness while marathon training

I know this has been touched on a few times, but I have my own specific question.

I was planning on spending the winter building my FTP, but I now have the opportunity of running my local marathon. I’d like to think I have a good chance of PBing, so I’m going to put my bike training on the back burner until then.

I’m planning to do two rides a week in the hope of maintaining my bike power:

  • 45 minutes VO2Max workout
  • 45 to 60 minute sweetspot workout

I feel confident I can handle those 2 workouts on top of marathon training. For example, I would do the VO2Max workout in the morning and a long easy run in the afternoon.

Any opinions on whether or not this seems reasonable, and / or guesses on what percentage of FTP I would likely stand to lose?