Re: 'Long Drag' 40M in 20 Hrs..
I've done similar stuff in the past including a 100 mile distance in 48 hours (although the terrain wasn't as mountainous). Are you going to be carrying the same sort of Long drag weights on your back? If you are used to walking, keep yourself fairly fit and have the right mental attitude the distance and timings will provide a challenge worth bragging about even if you're only carrying a daysack with lightweight overnight kit, emergency equipment, food and water.
55lbs is supposed to be representitive of the sort of weight likely to be carried going into battle and will definitely require a careful training build up. Shin splints, twisted ankles, knackered knees and stress fractures are all common side effects of this sort of training and I've had all of them in the past (if you enjoy getting out into the hills any of these injuries will be incredibly frustrating).
So my advice -
1. Don't overtrain and allow enough time for muscles to rest and grow back stronger.
2. Allow enough time between now and the challenge to build fitness and leg strength slowly. Don't be in a hurry and attempt it under prepared
3. Don't neglect your upper body training as you'll need this for carrying crazy weights on yer back
4. If you're doing this off your own back then recruit someone to provide back up for you. When you're pushing yourself to the limit you'll need to know that if anything goes wrong you've got someone to bail you out
5. Develop a good blister treating routine that works for you. I don't know if I've got unusually soft feet but anytime I've done long distance stuff I've got bad blisters after about 25 miles. Draining the blister, applying a padded plaster (Compeed) with zinc oxide tape over the top and (after 20 miles of walking on them) some good painkillers do the trick for me.
6. You're putting yourself under extra physical and mental pressure in a potentially dangerous environment so make sure you know what to do, and are properly equipped for a worst case scenario.
7. If you don't already know how much food and liquids you'll need and what types are best for this sort of thing then do a bit of research. It's amazing how many calories you need for 20 hours of hard slog.
8. Don't park near the Gurnus estate in Merthyr unless you know the number of a friendly taxi driver.
Hope that helps and hasn't been too egg sucky.