My mum does pleasure rides up to 20 miles. The biggest problem with stabling them, then doing pleasure rides is they see grass and turn into nutters. To prevent any such grievances we turn him out for at least a few hours the day before. To get him fit, he is schooled 3 times a week and does 3 1-2 hour hacks a week. He manages fine with the rides themselves, TBH as they are pleasure rides fitness isnt a massive worry as you pick your speed & distance!
To add onto the above post, it also depends on how quickly your horse fittens...some are quicker than others...my Welsh takes a while for example and only really peaks after doing several pleasure rides.
You want to build your horse up slowly until you are doing plenty of trot work out on hacks. Try to get him/her on different terrain also as this is good for balance and keeps them alert.
You need to ideally build up the distance a bit so that before you enter an event you are three quarters of the way up to the distance of the ride and have already done a ride of that distance. So if you are aiming for a 15 mile ride, make sure you have done at least one 12 mile ride before the event.
You don't need to worry too much about canter work unless you are planning to whizz round! But I would want to see a horse do 2 rides of a couple of hours per week as part of it's fitness routine. If only because adrenaline on the day can hide a multitude of problems if your horse is not fit enough!