.i donít want him to have more energy. He already has plenty for what we need him to do.
You can't have one without the other. Humans that work out will attest to that.
If that is Harley in your first post, he looks like he has gained weight since you bought him:(
He needs exercised as much as possible but not to the point of sweating him up or exhaustion. He needs to burn those calories off.
As others have already commented, everything depends on the individual horse's stamina, age, overall health.
For example, my 22 year old TWH has gained about 40 lbs since the loss of my two senior horses. That is because he won't cover this entire 23 acres by himself.
But he is still in condition to:
1. Take a slow hack around our country block that is ten miles.
2. Or go the 1.8 miles down hill to the end of my road and, with any sort of luck, be able to stay in his running walk for about 1/3rd of that 1.8 miles uphill
The best way to build lung power and strength is hill work. Just like human athletes, start out easy.
But the bottom line is don't expect conditioning without having more energy. They are synonymous no matter the critter or human:)