Training in the UK for the London marathon will normally take place in the cold wet winter months for the race in April. This prevents most people going out, especially in the evenings when it gets dark so early.
Try and structure your training around your current commitments. If you can, train in the morning and / or at lunchtime. If you have the time, and feel that you can train three times a day, then break it up so that you may run in the morning, swim at lunch time, and train in the evening on a stationary bike / rower.
Avoid doing speed work on a day like this, and again don't push hard on all three training times, if you are going to train hard, do it when you most like to train either morning or night.
Prepare the body with the fuels you need. The body has more than enough energy stored within it in the form of carbohydrates, fats and protein to be able to accomplish more than just one marathon, however it is best to have your carbohydrate stores, in both your liver and muscles fully loaded. The most easiest way to do this is by taking a sports carbohydrate drink and mixing to their requirements anywhere between 3 / 5 days prior to your marathon.
Always experiment with different brands so you can find one that you like. It would make sense to try and use the one that the feeding stations in the race will have, as you will need to drink both water and a sports drink during your run to avoid both dehydration and keep the body fueled.
Avoid taking sports drinks that you are not used to, as your body may want to reject it, (throw up) or it could simply give you uncomfortable stomach cramps. Practice when out training taking in both fluid, and if you can, small pieces of food, (bananas / sports bars), and this will help you avoid that phrase, hitting the wall.
Now lets look at your beginning to training for your first marathon. The first thing you will need to do is enter the race, this may seem simple, but not everyone who applies is guaranteed an entry form. Look in running magazines as these will help you choose a race close to home, or like the London marathon, have various allocated slots for persons wanting to run for charity. First step out of the way, lets begin training.
Now that you are ready to go out and run, make sure you warm up and stretch, see the section on stretching for more help and advice.