I would suggest starting of with small relaxed trail rides where you and your horse can have fun, and then start to make them longer with more trotting and transitions. After your horse is at a certain level you can then start slowly working on your preferred discipline.
When you're getting your horse fit you have to remember you want to build them not break them. So you want to start out slow like walking for 30min each day and backing them for a couple of weeks. Then you can start trotting and then loping and so on and so forth. It's important in my mind to lunge and stretch your horse. Riding daily also helps improves your horses mood, riding performance and helps connect you and your horses bond.