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Do you mean the Action Challenge London2Brighton? I've done it twice and it is very well supported - loads of snacks etc. and a massive great meal at the half way stage. The route itself is very much a 100K of 2 halves. The first is undulating - mainly through suburbs so it can be quite hard on the feet. The second half is trail and can be very treacherous if it has rained heavily. There are lots of stiles which can be tricky to negotiate on tired legs, not to mention the final infamous steep ascent to Brighton itself. If you are doing it as a runner and have a drop bag half way then you might consider road shoes for the first section and trail for the second.
The only down side is that there are no proper changing facilities at the finish and trying to change in a very cramped lavatory cubicle is not very easy - although they may well have done something about this by now. It might be worth checking with them - all the need to do is screen off 2 areas for male and female changing and put a few chairs in them. There is loads of room for this.
I actually walked it with a rucksack as I feel happier with spare gear. There is a wide range of provisions at the checkpoints - so you can take some spare items if you want to keep you going to the next.
The route was very well marked with glow sticks and fluorescent arrows for the night section. They insist on a form of buddy system after dark - which is understandable, but a bit annoying if you're like me and would rather go it alone. Some Cps just let you leave with another person - you can split as soon as you want. Others insist you leave with an official group - which leave at 15 minute intervals. However, even then the group breaks up very rapidly and you can go at your own pace from then on.
The route is through open road and country so their are no restrictions against non participants joining in at certain points. Friends and family are positively encouraged at the CPs. You will be able to download a route map nearer the event that will show where the accessible sections are.
I really fancy this too, although I can't make this years dates so maybe next year.
looking at the map on their website it looks to be 80% on roads? Some small country lanes, but not much trail, or am I reading it wrong?
I found the terrain extremely challenging - climbing over fallen trees/roots in the middle of the night. I'd trained off-road, but some of this was really hard, for a city girl like me, anyway.
The signage was brilliant, however - with glowsticks everywhere at night time, and loads where there were difficult hazards to get around.
My son did the last 5k with me, no problems ... even when it went onto the race course at the end.
Re: train stations - at one point the route actually went through one, although it wasn't far from the end by that point, so you'd probably want to keep going!