Back when I was seriously competing in endurance I rode my horse 3 times a week. Starting with about a 6 mile loop with a few hills. Once they were ready to progress I added a longer loop on the weekend. It was about 12 miles with a few much longer hills. We stayed with that routine of 2 shorter rides during the week and a longer ride on the weekend until soon the 12 mile loop was a piece of cake. Keeping in mind you want to teach your horse a good pace. Start by walking the hills, trot and some cantering on the flats. With that training routine my horse was ready for an easy and fast 25.
People think that all this extreme training is needed to be successful and competetive in endurance and its really not true. When I bought my horse the people who owned him were overconditioning him putting in over 60 miles a week and not giving him the time off or extra nutrition he needed and then wondered why they couldnt get him to complete a 75.
Even when I was competing in 100s I never conditioned him more than 3 times a week. As for working into more tough terrain you don't want to overload them. Change one thing at a time. Wether it is speed, more hills, or mileage, one thing at a time, just like for people. As for giving them a mental/physical break that really depends on the horse. My boy was born for the trails. It is his happy place. His ears are forward, he wants to go, looks forward to me coming and getting him. For him he never needed a break from it. Though sometimes I would hop on him bareback and go ride around the nieghborhood just for a change of pace. And for time off my general rule was after every 25 competetive miles we did he got a week off after. So if we did a 50 mile race he got 2 weeks off etc.. That again varies by horse. After a 100 he wouldnt always get a month off because sometimes he wouldnt need or want it, which I could tell as he would tear the barn down bored.
Again this is just what worked for me and my horse in my years of endurance. And I just couldnt help but share a picture of my superstar