Train the way that is best for you - this will require some experimentation, but once you find out what's best for your body, stick to it.

Just going out running the odd double marathon may not be the best method of preparation for a multi-day race.

Everyone's different

Mark Cockbain, a friend I ran the Marathon des Sables with, built his training plan around running numerous races, mainly half and full marathons, almost weekly. Frequently running in organised events helps with motivation and gives you little stepping stones or visible targets to the main event.

However, I would have found this high road mileage too stressful and think it would have induced injury. Though our preparation was slightly different, we both posted similar times. Everyone's training needs are different - experiment early on and find out the best way YOU train. Then concentrate on your own training and, while staying flexible, not worry about what others are doing.

Fitting it in

With life being as hectic as it is, few people will be able to train perfectly. Finding enough time to do the necessary volume of training for a race that is months away, or having access to a climate controlled chamber containing a treadmill are unrealistic dreams for the majority of competitors.

To improve your physical fitness I would advise building up your endurance base not just through running, but by combining running with cycling and/or rowing. When training you might want to consider the following components:

Running endurance
Running speed
Running strength
Rucksack strength

When planning my training I separated it into these elements, working on the strength and speed elements once a week, with long endurance work (e.g. cycling for 6 hours) done once a fortnight.

Running Endurance
For stage races, look at the typical distances involved, target the average stage distance and use it to form the base of your training. Take careful account of the climate and terrain.

For the Marathon des Sables (MdS), the typical stage mileage is around marathon distance. However, because I'm used to an English climate, not Moroccan, I roughly doubled my expected times for the distance. For example I estimated a marathon would take me 3 hours, so based my endurance training on 5-6 hours of continuous exercise.

For most runners the thought of having to run for 6 hours straight is hell. The easiest way I found to prepare for this was by combining other sports to accumulate the necessary amount of exercise. For example if you're not fit enough to run for 6 hours, you will find it easier to cycle it.

Once you have built up an endurance base of 6 hours on the bike, cut the cycling time down and do a run after it. While training for the MdS, I would cycle for 4 hours and run for 2. As your legs will already be tired/worked, this will also simulate running the second half of a 40 miler without subjecting your legs to such high mileage. This should help prevent injury and strengthen other leg muscles not exercised by running. Running speed

If you are used to running for 10-15 miles at pace, the slower speed that you will undoubtedly be running at on longer distance runs will it feel much easier and you should be more efficient.

Running strength

If your targeted event involves running up and down hills or over difficult terrain such as sand dunes, where the sand almost splashes away with each step, it is going to be extremely energy sapping. 

To improve your general strength for running over tough terrain, you will need to incorporate a hill running session into your training programme.

Here are some sample strength sessions:

1. Variable incline treadmill run
Find a comfortable pace and after 2 minutes increase the incline 2% every 30 seconds until you are up to 12%, then return it to 0%. Continue running on the flat (0%) for 2 minutes and repeat the incline raise.

2. Maximum incline treadmill run
Set the treadmill at its maximum incline and do 1 minute hill sprints with a 30 second rest. Repeat this 10 times.
  3. Hill sprints
Find a hill in a local park. Run up it, then jog down. Repeat this at least 5 times. Then after an active rest (i.e. running around the park), repeat again.

4. Run around a ploughed field
If you are running over rough rocky terrain this will be exceptionally hard on your ankles. Concentrate on improving your ankle strength for example by entering some cross country races, running around a ploughed field or running on horse tracks.

Rucksack strength

Many runners suffer from a very stiff sore back, sometimes very early on in the event. By doing some cross training, for example circuit training or rowing in the gym, you will strengthen your shoulders and back, getting a great CV workout at the same time.
  Typical sessions might include:

1. Try a cross training (or circuit) class at a gym
2. Rowing intervals. Row 3 minutes 30 seconds at 80%MHR, followed by 30 seconds rest. Repeat 10 times.
3. Run your long runs with a rucksack. Start light and work up to the weight you expect to race with.

Acclimatisation

If your target event is in a climate different to the one at home, you will want to prepare for that as best you can. Some believe that unless you exercise, recover, sleep and eat in the climate that you'll compete in, you can never truly acclimatise to it. Even if that's true and you are unable to acclimatise perfectly, you can condition your body as best you can for the conditions and, importantly, get the mind used to the discomfort. Some of my fellow runners preparing for desert racing trained on CV equipment in a sauna - the rest of us just went running with woolly hats on in summer.