Training
I'm a recreational runner, not a serious athlete (to state the obvious!). I normally run 3 or 4 times a week, a total of 15 to 20 miles. To run a marathon, you have to up your weekly mileage to at least 40 to 45 miles. And that mileage needs to include at least one long run a week, ramping up in length progressively to at least two 20-mile runs.
Training Update: 11 March 2012
I've completed about 380 miles since I started training in mid-December. My longest run to date has been 19 miles. Today I competed in the Silverstone Half Marathon, which is the official warm-up race to the London Marathon. Running round the F1 race track in glorious sunshine was absolutely terrific - loads of space and a lovely smooth surface. I finished in 1:46:39, shaving 1 min 26 secs off my previous personal best for this distance and coming 13th out of 98 in the men's 55-59 class. Better still, my average heart rate was 7 bpm lower than it was in my previous fastest half marathon (Bristol in Sept 2011). The training is clearly working.
Training Program
I'm following a pretty standard 18-week training program. During the program I'll run at least 550 miles. Sounds a lot, but that's what it takes to get ready to run 26 miles straight.
It's very easy to over-train for a marathon and put so much stress on your body that you get injured. Apparently, many more people fail to make the start line of a marathon than fail to make the finish line. The best way to control the intensity of your training is to monitor your heart rate. I run with a heart rate monitor and stick strictly to a formula that is supposed to maximize your aerobic function and hence your endurance. This means running at a relatively gentle pace - about 10 minutes per mile. I expect to run quite a bit faster than that on race day.
One of the terrors of the marathon is "the wall" which many people hit at around 21 or 22 miles. Your body runs out of glycogen, which is the fuel your muscles normally rely on, and your legs just give way. One way to counter this is to train your body to burn fat rather than glycogen when you run. You do this through a combination of lots of aerobic training and by eating much less carbohyrate than normal, but plenty of protein and fat instead. Bacon and eggs for breakfast every day!
The timing of the London Marathon, in late April, is not exactly ideal for training. It takes quite a bit of motivation to go out and run 5 or 6 miles on a cold, dark, dreary, damp morning in January.