Depends how much running you've done imo, if marathon distance is still relatively new then recovery between days is going to be slower. Also depends massively on what you want out of it, if you're just there to enjoy then take it easy and enjoy, only you know what that pace means. If you're looking to push yourself then I would make progress on day 1 while still fresh, but be aware of situations which may affect recovery, e.g. take special care on steep hills to preserve quads. Maybe start out at your weekend long run pace and take it from there.
In my experience the first mile or so of days 2, 3, ... will be a little sore but once properly warm you'll be fine.
It probably helps a bit if you know what terrain is coming up, no point pushing it on the hilly bits only to be tired when it flattens out, having said that most of the Ridgeway is very runnable.
agreed with pmo, though if you're treating the races as long runs go crazy, and I'd def take it easy on the 50.
A good quote from Stuart Mills goes something like "The minimum number of miles needed to complete a 100 miler is the number of miles it takes for you to believe you can do it".
I thought that was great, if it takes you 70 miles to put you in the right frame of mind then run 70, if you're happy on 30 then great too. What you don't want to do is run 30 and believe you needed to do 50 because that doubt will eat away at you and give you an excuse to quit when things start hurting.
From a runners point of view it's logistically a very easy way to attempt such a distance. From a race organisers point of view it's 3k for a weekend's work, it's not something I think I'll ever be interested in but he only needs to find 30 people who are and it's mission acomplished.
For me the Irontrail distances mean it will never be as popular as UTMB, 200k is a serious undertaking in that environment and 140k is just a bit awkward. 100k and 100m are classic distances which will always draw people in.