To be honest I've never really thought about it. Yes of course it is a race in the sense there will be a first one back, etc.
It's a 23 mile walking/running race, you're given a route description and there will be checkpoints providing food and drink about 5 miles apart. It'll be of the sort "turn left at crossroads and cross the field and over the next two stiles and then left down the track." My Dad and I did it one year as an event and forgot our map, we asked others to look at theirs and most people didn't have one. I just like it for peace of mind.
There'll probably be more than 100 on the event, chances are you won'r be very far from another person, most often you just follow someone else.
It's put on by the Long Distance Walkers Association, and I would call it a challenge walk (although lots of people run them) and part of the challenge is the navigation, there won't be marshalls at every turning pointing you in the right direction.
If it puts your mind at ease I don't think we've ever lost anyone yet and each participant is checked off as they reach a checkpoint.
R