You have to decide that you're in it for the long haul. Gradual changes to diet and exercise regime over a decent period of time will be the best way of doing it. Just dropping everything you're used to and throwing yourself into running (and expecting to be as fast as you were) will inevitably lead to failure.
You've identified the issues (lack of run training, diet, weight) so now you just have to tackle them sensibly. Being pretty fit anyway (I'm guessing) and having been a decent runner in the past, should mean that it probably won't take as long as somebody starting from the couch.
I've only done two: Purbeck and London (about as different from each other as you can get). I'd do both again and will probably try again for London in 2024 (10 years after my first).
Not mega keen on road marathons but am doing the first Bristol to Bath Marathon in October and would quite like a crack at Snowdonia one year because it sounds flipping hard! Medoc sounds fun but I just can't see how heat, red wine and marathon running could possibly end well!
Trail wise, I'm doing Stroud in 6 weeks, which is a local one to me and quite fancy the look of some of the endurance life coastal ones for future years. I'll be sticking mainly to trail races unless something special catches my eye.
Don't discount trail marathons Andy, there are some really beautiful ones out there.
Car parking and shuttle bus email arrived this morning. I'm booked into a B and B near the finish for the night before, then have parking at the finish and a shuttle from Avebury to the start booked for 5am on the Sat morning.
I have an estate car so am planning on throwing a sleeping bag and mat in, and can then sleep in the back if I find I need to before driving home. Being as I expect to finish in the middle of the night, I think it's likely that I will need it!
I do see people selling running gear but would probably only buy any of that if it was quite heavily discounted and it never is.
Only thing I've ever bought before a race is a coffee. I'd probably buy stuff I actually needed for the race ahead if I happened to have forgotten it (gels, plasters, vaseline - all for longer races above 1/2 mara).