You go to a ride and vet in, Lets say you are doing a 25 mile LD, You start at 6 am, go yah muling for 12.5 miles and come into your first vet check, from the time you come in you have 30 minutes to pulse down to around 60, then the vet checks out your horse and judges you fit to continue if all is well and you start your hold at that point. Usually around 30-45 minutes. The hold time doesnt count as part of your ride time, So you head back out and you still have to be completely done and pulsed down at the final vet check at 1230 in this case.... Six hours total for a 25.,,,, So do the math, if you ride 6 mph , you will have a ride time of 4 hrs 10 minutes, that leaves you 1 hr 50 minutes, 30 minutes to pulse down at the first check, 30 minutes to pulse down at the end, and usually about a 40 minute hold,, that only gives you about 10 minutes to spare..... No ideally you shouldnt take anywhere near 30 minutes to pulse down, so that saves you some time.
I am a firm believer in train like you fight. If you are going to need 6 mph to finnish that is where you should be training. But you have to pay attention to your horse. I started endurance with a drinking horse. Although he had never been asked to do much at speed, he had several years of party ride experience which consisted of 12-18 mile trail rides at probably 3-4 mph. SO he had a decent base. I just started picking up the speed. What worked for us and got me my first 50 mile Leatherwood Mt completion, was riding a bit over 7 mph. When my average got over 7 I stopped for water and grass, and basically let him eat and drink while I paid attention to my average speed, once he got down around 6 I moved on out again. Did that all the way in and counting the 20 or so minutes to pulse down at each check ended up with a 9 hr and some change completiton on a fit to continue horse.
Listen to your horse get some miles on him, head out for 2-3 hour rides and don't worry about the speed or distance. Just keep him moving.