Gayle Ecker, one of the foremost researchers on equine exercise physiology, strongly advocates periodic "fitness testing" to see how your horse is doing--getting stronger or weaker, etc. The way she did it was on a treadmill; which is impractical for most riders, however, she sent an EXCELLENT program that can be done on the trail. Do this fitness testing before the scheduled Endurance ride, give the horse a couple of weeks off, and do it again. That will give you a more quantitative measure of how far the horse has recovered.
Fitness Testing, from Gayle Ecker
Recovery heart rate test
Need a stethoscope and stopwatch or heart rate monitor. Measure off a known distance that will remain fairly constant in footing from one repetition to another. A 5 mile section of trail, a one mile gravel road up a gradual slope, a 5 mile distance around a series of fields. Use a ATV, dirt bike, or vehicle to measure off the mileage (or kilometers).
After a warm-up of 15-20 minutes that includes walking, trotting, a bit of cantering, until the body temp of the horse is warm, then start the fitness test. Trot your horse over the distance and using the stopwatch time your duration over the chosen trail. You can trot some, canter some, even walk some, according to the fitness level of the horse. The overall intensity/duration should not be harder than your general training miles.
At the end of the distance, record the time to complete the circuit in a little notebook that you carry in a pocket. Start the watch again at the stop of exercise. At 2 minutes, take the heart rate using the stethoscope for 15 seconds, record this number. While dismounted, walk the horse along and re-take the heart rate for 15 seconds at 5 minutes, 10 minutes, and 15 minutes. Record these numbers. Cool out your horse (or continue with training miles, depending on the fitness level and your targets for training).
Once back in the barn, construct a graph (purchase graph paper from a stationery store to make this easier, or use a program like Excel if your good with computers):
On the bottom of the graph (the x-axis), put the time (0 to 15 minutes).
On the upright axis (the y-axis), place Heart Rate.
Record the heart rates above the appropriate time (HR at 2 minutes, 5, etc.). Connect the dots. Note the slope of the line.
Repeat the same test periodically, keeping the distance and duration as much the same as before. In other words, you are testing recovery heart rate, not the speed at which you can complete your circuit, try to aim for the same time as prior tests. If you start this now, and repeat it periodically, you will have an objective measurement of how much conditioning has been gained or lost based on how quickly your horse recovers from a set level of exercise. Keep in mind factors such as temperature and humidity will also affect results. Best to record the date, temperature, and humidity as a footnote for each session if possible.