Its about priorities but you will hopefully have the benefit that the tri training will reduce the urge to overdo the run training and risks of getting injured.
I hope to do something similar, hopefully a fast marathon in early April, maybe the same at London a few weeks later, and then focus on the tri training and aim for a decent HIM in June. Your recovery will be key.
Congratulations, its a great event and like all marathons harder getting to the start line than finishing it. I've had both stress fractures (out for 2 years) and various knee injuries but thankfully seem to be fine now and run 4 marathons in the past year. My regime these days includes:
A daily ibuprofen tablet to reduce inflammation
Plenty of milk to ensure bone creation
Flexiseq cream for sore knees
Run as often as you can on grass and avoid as best you can road/ tarmac
Plenty of rest, esp when you know your close to the line of getting injured
Fish oil and glucostomine tablet daily
Don't overdo it and set yourself a reasonable goal
On my 2nd marathon London in 2007 (3.00.13) got overtaken in the last 800 metres by a load of guys chasing the sub-3 pacer who were debut marathoners and they sub 3 (o how I hated them!) so it certainly possible.
If you hit the following milestones, you increase your chances:
Aim to run faster maybe a half marathon time of 1.24-1.25
Build endurance - At least 4 runs in training of 20m plus and make 2 of these progressive, i.e.faster second half than first.
50 mpw, the more the better as long as you don't cross the getting injured line
Lots of sleep and recovery
Then a fast course on a cool day - April in Manchester must give you that.
TJ - Also a newbie and just done Gower Olympic on Saturday - me: good runner, ok cyclist, below average swimmer - and looking at the Cotswold 113 / Classic events in summer 2017. Offers a lake swim and flat bike and run so should be 'easier' than most.