Attended an excellent lecture this winter by Gayle Ecker, one of the
foremost researchers on equine exercise physiology. She strongly
advocated periodic "fitness testing" to see how your horse was
doing--getting stronger or weaker, etc. The way she did it was on a
treadmill; I emailed her that that was impractical for most riders
and she sent me an EXCELLENT program that can be done on the
trail. I am going to cut/paste it at the end of this message. I
think (as per Kim's sage advice)--do this fitness testing before the
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.
At the 100 I co-manage last year, six horses/riders finished; all
six horses were first timers and four of the riders. Two of us
experienced riders helped the new100s through. There was a 50 mile
ride two weeks later managed by the other experienced rider. Both
she and I had no intention of riding that 50. Two of the
first-time-100s entered; 1 didn't get to start (poor CRI and
metabolics at check in!). The other rider completed, but around the
35 mile mark his horse hit a wall--something it had never done
before--and he had to pretty much walk/jog the rest of it. Before
the ride he felt his horse was "great"; after the ride he told me his
horse was "spent". I think a tool like this fitness test might have
prevented that.

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 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).
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, keeping the distance/duration as much the same as
before. If you start this now, and repeat it in the spring, you will
have an objective measurement of how much conditioning has been lost
over the summer, and then you can track the fitness gains over the
season next year.