I have a Horse-O-Meter and have never been able to set it up properly!!! I got a GPS for Christmas and track all of my trail rides with it. They are generally in the 5-8 mile category. I ride Biscuit almost every weekend at least once, sometimes twice, and it is mostly the 8 mile variety. Lots of walking and trotting - been working on his jog which he is doing a great job at. I have taken him on weekend rides and rode 25 miles or so and he has done just fine. One weekend he got tired and decided he had trotted enough and that was it - it was a walk for him....he is a smart boy!
Just get out there and ride for several hours on the weekends and some during the week if you can manage it and he should be fine with a walk/trot weekend. Have a good time and take pictures!