I've done a pleasure ride 20 something kilometres) in endurance and also helped at a few which is a great way to get to know the people and learn more about it, hoping to do more this year with my Haflinger mare ( she had an illness and then a major op for me interrupted our plans).
It really is very rewarding and although some participants are scarily focused with goggly Arabs, most are on Heinz 57s with normal tack and just enjoying working towards the rides, it makes hacking more fun the rest of the time if you are fittening up for a ride.
To do a pleasure ride within the min/ max time you would ideally want to be able to hack on varied terrain at an average of a reasonable trot all the way (ie any walk walanced with equal distances of canter). However there are sometimes smaller distance taster rides which are only 10km and which any horse in regular hacking work could do.
If you are doing your first ride and don't have someone to ride with, contact the ride organiser and they will likely be able to buddy you up with someone to look after you. They can also likely lend out the few bits of kit you need eg medical kit, armband, map case.
My neighbours both ride, husband does endurance seriously on said goggly Arab . Wife rides a fell pony and we hack out and she does occasional pleasure rides on her Fell mare.
I bet young Jimmy will have a great time!