Three months at 30 mile/month is not very much at all and you risk an injury going from that to 40/45 m wk too soon.
Can you do a Saturday parkrun once a week or fortnight to keep some semblance of speed going and then alternate 90 to 120 minutes on Sundays? On say one of those longer runs each month, do a few miles in the middle near your marathon pace (start with 2 or 3 miles and build up).
Can you get a pass for a couple of early morning runs in January to ease back in before February......?
If you have almost 20% fat then that is about 1/3 too much. In my case I am at about 13%.
I trained as a mountain biker for about 9 yrs (including racing) before switching to running - I thought I was fit, but by running long and slow three things happened:
1. I lost the bulk on my thighs which you don't need for endurance events
2. I lost 7lbs without really trying
3. I converted my energy system from high glycogen dependency to fat burning. If you have "high cardio" fitness, I suspect you have the glycogen burning engine (think Ferrari, whereas I might be able to outlast you in the hills with my slow chugging diesel).
So on your ship's treadmill, you may go crazy staring out of a porthole, but just chug away very slowly to convert your energy system. If you are going somewhere warm, stay hydrated and remember that 1kg = 1 litre of sweat.
Do at least some of your training in a fasted state (no food) so that your body draws on fat reserves first, but you must stay at a "chatty" pace - this is not about lighting the afterburners if the guy next to you is on an ego trip.
Most fell runners are skinny and light for a reason so you may have to change your body shape if you want to do more than "get around" or is "up & down"?
Good luck with it....you could do worse than hop onto the HADD thread....,