Oh my -- that's an old horse. He may be giving you his absolutely best, but those old muscles and bones just don't work like they once did. Just like people, animals get sore. The older they get, the more pronounced it is for them. Even after a seemingly normal day, they ache and stiffen up, just like people do.
I know how much I've slowed down in my 50's. I just don't have the stamina that I once had, and I certainly don't have the speed that I once did. I'm old. It's expected. I can't work like I used to, and an old horse like that certainly shouldn't be expected to either.
Be kind to the old fella --- he's in his 80's (in equivelant human years). Let him give you what he's able to offer, but don't beat him or work him too hard. He's a senior citizen who has earned the right to move at the pace which is most comfortable to him.
I heard about a woman who had a little pony cart hooked-up behind an old pony. She was a fat lady and the horse was clearly not big enough to pull her. But every year, she'd trot that poor horse out to the big Julian CA parade and would force that old horse to drag her fat ass down the parade route. A couple of years ago, the horse had a heart attack and died, right in the middle of the parade. True story. She'll never live that down (Julian is a small town where everyone knows everyone).
Be kind to the senior citizen.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf
Sorry Grace, but I have to agree with Marco. Danie is in his retirement years and while he might have an occasional frolic in the pasture, he just isn't going to be able to carry the weight of a person at any speed. It is his time to be pampered for all the years of service he has already given.
"She paid for who she is with her life, but don't we all?" ~Alice Walker
Why does it matter that he go fast? Are you competing him, or just riding for fun? Or are you using him on a farm? Devil's claw is a great joint supplement. I agree that he's quite old, so don't expect much. Is he a good weight (not skinny)?
Hey, sticks and stones baby. And maybe a wee mention of my stuff:
Devious Experiments for a Truly Passive Greenhouse!