Zee was off for about 5 years before I put him back to work last August (he'll be 22 in May). We're now doing 2' and getting ready for the upcoming show season!! The first thing I did was have a full vet check. I did senior blood work, and had a physical exam to make sure he was healthy enough to go back to work. I also changed his supplements and his diet to ensure proper nutrition. Plus a massuse and chiro visit. When we started exercising, we did about 30 minutes 3 times a week. Mainly walk/trot with lots of breaks and stretching. Each week, I would introduce something new to wake up his muscles. Bending, poles, stronger canter work, etc. I listened to him very carefully, and any time I noticed him getting stiff, sore, or cranky, I'd give him a few days off. I'm a firm believer that senior horses need moderate exercise just as much as any other horse to keep arthritis and other changes at bay. He'll let you know when it's too much, but it sounds like your plan for him would be very beneficial to his health and enjoyment. I know Zee is VERY happy to have a job again!
I've spent most of my life riding horses. The rest I've just wasted.