Adapted marathon training plan with significant cross-training?

Hi! I’m aiming to start a 12-16 week training plan for a mid-April marathon, but I’m recovering from injury (what I believe to be medial tibial stress on my right shin). I’m working with a PT and am able to run 2-3 days per week (elliptical on off days) while building up mileage again. I’m wondering how feasible it will be to start marathon training while doing up to half of my mileage on the elliptical–– any advice on adapting a training plan to incorporate a high volume of cross training? Thanks in advance!