Here's what I learned about the Rest-Less skill...
Because your horse uses less energy during each train, you don't need as many meths...
HOWEVER, horses only gain in Endurance when they are tired enough to really have to push themselves. If you are familiar with how we used to use diet to make our horses tired-er as their Endurance levels increased, then you know what I am talking about. We had to get them exhausted by essentially starving them (yuck) before they even started training each turn.
With the Rest-Less skill, your horse doesn't get as tired as fast (good for using less meth), so your horse takes longer to get tired enough to push themselves (bad for increasing Endurance scores).
Eventually, when your horse's Endurance score is high enough, with the Rest-Less skill your horse gains more energy than it uses each train! You will see the green energy bar get longer!
So I experimented with how to still raise Endurance while having the Rest-Less skill. I found two ways that really helped.
First, do what you know, only sooner. Use lower diets as soon as Endurance stops increasing each train. You will have to do this much sooner than you are used to, and you will have to go much lower on the diet than you used to. In fact, by the time one of my horses reaches the point where it has a net gain in energy each train, I have my horse down to a total diet of about 10-12%.
Second, change up your training. We are all used to doing 6x of this per turn until this stat reaches 200, then 6x of that per turn until this stat reaches 200, etc. Change it up a bit. Let's say you have a horse you are training in Dressage. While your horse is a Halter horse you need to train Obstacle and Gaited, of course. Normally we do 6x Obstacle until Intel maxes, then switch to Gaited until that maxes.
Well, Gaited uses alot more energy per train than Obstacle does. You can reduce your use of meth during Gaited training, AND improve Endurance gain while using Rest-Less, by reorganizing your training like this:
From 0.1 to 1.0: Train Obstacle 6x per turn
From 1.1 until Intel maxes: Train Gaited 1 or 2 times (2 times if you don't get a gain on Endurance the first time) then Obstacle the rest of the turn
Then Until Gaited maxes: Train Gaited 6x per turn
Why do I only train Obstacle for the first year? Convenience. You can't switch between training types easily during the first year (you have to go back out to the stable to do it).
After that, I train Gaited first during a turn, to use up enough energy so that the rest of my Obstacle training in that turn will also gain Endurance.
By the time I get to straight Gaited training, Endurance is usually over 130 and so I can train 6x a turn without using meth.
This gives you an idea of how to reorganize your training (using multiple types of training in the same turn) to take advantage of the Rest-Less skill, reduce use of meth, etc. Use the same thought process for the training in any discipline (i.e. how can I combine training that uses alot of energy with training that uses less energy within a turn, so I can avoid meth and increase Endurance?).
In the beginning it will feel like you are trying to pull your brain backwards through a keyhole. BUT, I do find that eventually you can train your horses up faster (they stat max younger).
Good luck!