Google 'Trailwalker' and you should get loads of info.
I heard that the event had changed recently, takes quite a bit of planning to enter a team, you will need at least 8 'volunteers' as people drop out/go on tour/get injured and you will need an admin party to drive and secure your grips.
For the trailwalker you have to raise a minimum of Â£1000 and then theres the entry fee on top of that .
I did it a few years ago and plan on entering a team this year from 16 Sigs. Last time I did it we trained for a couple of weeks doing 12 miles a day but then got sent on exercise 3 weeks before it started and did no training for them 3 weeks, still did it, it was hard, especially the last 10K or so but it is well worth it.
Recommended at any price