As soon as you start talking about the marathon someone is going to say that it is the ultimate "endurance event". Of course they mean the race, but in fact the training is the most arduous bit and takes as much, if not more, patience and commitment than the race. That was certainly my experience during my preparation for the 2013 London Marathon.
It's hard to emphasise how much the training dominates. It cuts into every aspect of your life - you are either running or waiting to run! But, if you've put in the miles during training then by the time you get to the event you know you have the capability to complete the race. I found that certainty hugely comforting. Both when the butterflies were rolling around in my stomach before the start, but particularly when it started getting difficult during the race.
If you're a regular runner then the shortest period of training is three months, and if you don't regularly run then you should be planning on four or more. For the Virgin London Marathon 2013 we had a total of 18 weeks, which was fine - though I would have preferred 20 weeks.
The next decision is which training methodology will you use. There are a lot of training plans out there and plenty of opportunities for research! For those that want to focus on completing the marathon (rather than achieving a specific time) which was my objective my take-away is that your core focus should be on increasing your weekly mileage with regular runs throughout the week and a solid long run during the weekend.
For those that want to aim for a particular time then you need the above but additionally speed work is really important to adapting your body. I found structured speed sessions pretty difficult to do as most of the plans want you to be on a track. That doesn't suit me as I am mostly running as part of my commute - so I used a variety of Fartlek drills which did pay-off with increases speed - I should have done more of them to get the most from it.
It's hard to really keep doing the sessions every week but regularity is everything - so even a short run fit into your schedule is worth doing rather than skipping. That said if you do miss a session or so don't worry about it too much.
There are lots of good sites and magazines for running, but the marathon is harder so for a concentrated level of expertise I used two books: The Non-Runner's Marathon Trainer and Hansons Marathon Method.
The Non-Runner's Marathon Trainer is written by a team who run a marathon class at the University of Northern Iowa where they've focused on taking non-runners right through to running their first marathon. So it's really ideal for anyone whose starting right at the beginning of running. N loved it and bought it physically and as an E-book so she always had it! The training plan is pretty standard and really does build from absolutely nothing. Where it's a bit different is that it has lots of advice on the mental side of preparing yourself (such as visualisations) along with small vignettes from other runners who successfully followed the programme. At some points my British sensibilities found the sentiments a bit too twee and wholesome but these bits are easily skipped - And frankly I used the visualisations plenty!
The Hansons Marathon Method covers a coaching method created by the Hansons who run an elite training programme in the USA. There's a sort of orthodoxy on what training is required to run the marathon whether beginner or elite. The Hansons method is different focusing a higher frequency of runs (up to 6 times a week) but generally at shorter distances. The Hanson method also breaks from focusing on achieving a very long (20+ miles) long slow run - they believe the long run should be shorter (to prevent injury) but due to the increased sessions you run them on tired legs which aims to mimic the end of the marathon. The book does have a beginners plan for those doing their first marathon - but it's not aimed for someone whose just started to run. The expectation is that you've run a fair amount so the plans and language of the book assume prior knowledge. I found the material really good - it's very scientific and explains all the detail clearly; it covers all the information you need right through to race day and I thought the sections on race fuelling were really stand out compared to all the other books.
For various reasons I ran a mixture of the two books plans. I did most (not all) of the weekly sessions from the Hansons Marathon method, though I found it hard to fit in all six days and didn't do that much speed work. Since my partner was on a more traditional plan we continued to run the long Sunday runs at a distance further than Hanson specified - but I was doing fewer sessions so it evened out.
During the winter months it can feel particularly soul destroying as you come back frozen from another tramp around in the wind and rain. I found that fueling emotions and imagination in the form of on-line forums and some runners biographies really helped. I can whole heartedly recommend Keep on running: the highs and lows of a marathon addict by Phil Hewitt - you can guess the contents, but Phil is very funny and interesting as he explores why and how a 'normal' bloke becomes a serial marathoner.
I'll cover some other aspects such as food, sleep and yoga (yeah, I know!) some other time. But, for now the overall summary is that it's true that anyone can run a marathon - as long as you're willing to put in the four months of training, then four hours of running is tough but achievable!