Hilary: congratulations on your place, and on starting thinking about this now, not waiting until January.
Start running regularly now and gradually increasing what you do. I'd suggest starting with three runs a week, always with at least one rest day in between runs. Find an online "couch to 5K" programme and see how far along that you are - if you can run 3 miles, you're there already (5k is 3.1 miles) so you could already go find a local parkrun and do that on Saturdays - they're great, fun, friendly and free! (Just remember to register online beforehand and take your parkrun ID with you).
Once you've got to the end of a 5K programme, find say a 10-12 week half-marathon training program (they usually require you to be able to run 20-30 minutes in one go at the start) and follow that - try to find an actual half-marathon to do when you get to the end (a nice friendly local one, perhaps). Once you've completed that, start a marathon training programme appropriate for your speed. If you think it's too early to start the marathon programme, then start increasing the length of your long slow run, and work up to four or even five runs a week.
Note: once you get to a marathon training programme, I'd advise you to start it at least two weeks early - so, if it's a 16-week programme, try to start it 18 weeks before the marathon. That way, if you're out of running for a week or two at any point due to illness (e.g. bad cold) or injury, you're not panicking and worrying that you won't get enough long runs in (a theme that comes up repeatedly in the "Heath & Injury" forum).