I did the Witton ride/event last year on a full suss Rocky Mountain Element with Sammy Slicks, actually 103 miles before my knee collapsed long standing 24 years old injury
ave speed 10.2 miles per hour limiting factor was a double chain ring, and no lock out front or rear
made all the cut off points comfortably. Training was limited due to injuries in the lead up so really rode on training from Sept to Feb, 50 % hardtail and Full suss, lots of 70 to 80 mile rides
On the event there were two others on Full suss a cove and something else with big tyres
So yes doable and a challenge……….I was planning on a 29er HT for the event this year but was not luck in the lottery
Enjoy