I had some anterior tib problems around xmas. Yes, I substituted a lot of running for rowing for a week, but longer term, if you are not doing regular long runs then a marathon is a daft event to enter (even those that aren't London)!
What did I do? Regular icing - 4x per day, anti-inflamatories. When things settled, calf stretching, and then some strengthening. If I wasn't back up and running within the week I was going straight to my physio.
Luckily, condition improved while I knocked out half marathons on my rower - but this wasn't going to improve my running. Returning to running, I built it back up on soft surfaces (trails and treadmills), and very very gradually increased road mileage.
I still do a lot of cross training instead of running, but I don't drop the key running sessions (Long Run, Tempo Run, Mid week medium Long run, Interval VO2 session). All my recovery runs and easy runs are dropped for the rower which has taken out about 20miles pw from running. Hopefully this will reduce risk of injury reoccuring.
If the target is 4 hrs, and it is your 1st marathon, then deferral could be a good option allowing you to get fixed and start establishing a base mileage before going into a marathon programme. Then again, I ran my last marathon with a knackered leg, so there is no accounting for peoples stupidity