the endurancegb ones are great but you do need a fairly fit horse and rider to manage the average speed over an extended period. Or so my local contact told me - need to be able to do 10miles at 5kmph he said even for the pleasure rides? or did he just not want me dawdling!
look at ridingdiary for all the fun eg sponsored rides.
and theres even a specific website called funrides but not sure how complete it is.