Don't be lulled into not adjusting your speed for a long gentle climb. If you run yourself, you know how failing to slow down a little for a slight grade can take its toll. It is a common mistake for endurance riders not to compensate for a long gradual incline, so be sure to took ahead and pay attention. Trail conditions also affect the energy requirements. A rocky winding trail takes more energy than a straight smooth trail.
Soft footing takes more energy than firm footing, with deep sand and mud the most tiring of all.
Going too fast in deep footing also risks serious injury, particularly late in the ride when the horse is already quite fatigued. Watch the trail ahead, evaluate its effect on your horse, and adjust your speed accordingly. The only time in the entire ride your horse should run anaerobically is in the sprint to the finish line. However, to use a monitor wisely takes practice and knowledge of your horse - you can't ride only by the numbers from a book. A good horseman doesn't necessarily need any special equipment to know how things are going, because the horse communicates in many ways.