Marathon for charity. Never ran before

Yes, breathe through your mouth!

Rory, the main reason that I  discouraged using FIRST for someone new to running, who is going straight through to marathon distance, is that FIRST has a higher intensity level than any other plan I know,, e.g Week 1 Long Run 8miles at MP+30 seconds, Interval sessions every week faster than 10k pace, and Tempo runs as the 3rd session.

Although the pacing is not going to be fast, I suspect that most gains, and minimising injury will be made from easy running. FIRST is not the type of plan I would recommend to someone just starting out, who really needs to focus on mileage building, and has said they need to shed some weight. I've nothing against FIRST, it works for lots of people, but I think its a bad fit here.

Rory, I'd second the recommendation for the Hal Higdon plans.  Also-ran is talking sense about simply building up the mileage at an easy pace, and HH gives you a good straightforward schedule for doing this.  Good luck with it all, and hang around for advice whilst you're going through the training.

Well done on the lost half stone - diet is always the best route.

Just one last thing on FIRST. The authors of the plan only have a 5k Training plan for novice runners in their latest edition and they don't recommend marathon distance (but then no one does to novices). If you want to find out more, then the book to read is "Run Less, Run Faster"

Of course 26.2m can be done, and I am sure the mental battles you have been through will help

I would go with a more straight forward plan, like HH. Your objective is to raise money for the charity. Something like HH will minimise injury risk - you definitely want to be on that start line. 

Good luck Rory

Well done in signing up and starting. I ran the FLM (as it was then) for charity 5 years ago from a standing start. In a fit of New Year's resolution, I got a charity place in the first week of January and had to buy my trainers before I could train. My first run also 1.5m made me feel sick. I did a 16 week novice plan (and can send it to you directly so you can see what it looked like - given to me by a running personal trainer). I did, however, get overexciting and start doing speedwork and increasing distance to quickly and injured myself. I was quite lucky to get back out of injury to run at all and managed a fairly slow marathon. My lessons learnt is (a) if you are a working parent, assume you won't manage to stick to the training plan every week as work/children/baby no.2/tiredness etc will get in the way (b) that's OK - few stick 100% to a training a plan once you start asking around, for the same reason (so don't panic if life does interfere) (c) bearing that in mind, if you can, add in a small hadnful of extra weeks to allow for this - you sound on plan already (d) gently, gently, gently does it. Don't increase speed, mileage or long run distance quickly even if you feel great - what you can't see is the muscolar-skelatal impact until you injure yourself. Finally, when you get there, go out to enjoy that marathon and focus on geting round rather than time in that first. The London marathon has the MOST incredible atmosphere and it's just fantastic to run and soak it up.