Reading comments by Cmoi over on Strava reminded me that having a plan would be a good idea.
My target is to complete 72k in 11 hours. To aĺow time at the checkpoints, I reckon that I should train to complete in 10 hours. The event is on 21st August so I've 16 weeks to go. Currently, my longest run is 35k which took me 4 hours 20 minutes (which suggests to me that my target is achievable, provided I can keep going).
I tried looking at plans from Garmin and Strava but they don't go beyond 42k.