No way of knowing , you say you ride 4 times a week, but how far ? How fast ? How heavy a load ? Just to many variables. You need some kinda speed over measured distance device. Most of us use a hand held or wrist GPS. I am a turtle rider, IE come in back of the pack on endurance, so I tend to average about 6 mph over 25-50 miles. Dawns a front runner, she's probably averaging 9. But that's average, counting stops for water breaks and such.
What Id do to figure out whats what is come up with a way to measure pulse and measure distance/time. Simple math will give you speed. Sooo get out there and walk/trot/canter. Say 10 miles. Stop check your watch and check your rides pulse 5 to 10 minutes after you stop. If he is at 60 bpm, you didnt ride fast enough. Add more cantering less walking next time. As he gets in better shape you shouldnt have any problem cantering the 7 miles, and still pulsing down in around 15 minutes, assuming a reasonable level of fitness and a horse suitable for the task at hand. I mean if he's an over weight shetland and ur pushing 300 lbs, might have some problems.
Granted there are better more accurate ways to go about this, but Im figuring on you not buying anything, as you probably already have a watch with a second hand. You will just need a known mileage course... Also some cell phones have GPS aps you can use. But you'll have to ask someone else.
You just don't wanna over run your horse starting off. So learn to take pulse, can stop any time and go to a slow walk, dismount and mosy for a few minutes within 5-10 minutes pulse should be in the 60's .. All things relative I guess, those of us that train for 50"s 7 is just a warmup. But for walk along trail riders 7 is a couple hour ride. I've done 15 mile loops in under an hour. Granted my horse was pretty much done for the day, but he was none the worse for ware. Took him about 35 minutes to pulse down, so oops. Live and learn, A little to much YAH and not enough whoaaaaaa