Brief running history: Started running in 2003 aged 29 to try and get some weight off as I was a bit of a lazy porker to say the least (15.5 stone and prone to the odd drinky poo/pie/deep fried pizza). Gave myself three months and if I didn't enjoy it I'd call it a day. Turned out I loved it and entered my first half marathon in 2004 (finishing in 1hr 35), with my first marathon 6 months later (3hrs 37). Gone from strength to strength since then, getting my HM pb down to 1hr 23 in 2006 and marathon pb down to 3hrs 4 this year, not to mention losing 5 stone along the way..
Reason why we should train you: Despite all this I've never felt like I've done myself full justice in a marathon. Most predictors suggest a sub 3 should be possible, and recent training/races suggest this as well (sub 62mins 10 mile race a few weeks ago). I've never really used a proper training plan or had any sort of coaching, and maybe this is what I need in order to get under three hours.
Also bear in mind that with a proper taper you'll be starting you're marathon fully rested. Each long run you do is in the middle of a packed training schedule, so there's a big difference. If you live in an area with similar terrain to me you'll almost always find the event itself is being run on a far flatter circuit as well.