A hard surface has less "give" and allows the horse to get more "purchase" when loading and breaking over prior to lift off in the hoof. The downside is that it also increases concussive forces. The Big Lick crowd will sometimes induce a mild "road founder" by riding horses on hard surfaces as part of the soring process. This is one is difficult to detect.
I've heard for years that gaited horses will "copy" the gaits of companion animals. I don't believe it. If it were true we could put serveral dozen (if not several hundred) gaited horse trainers out of business in short order.
IMO this is an illusion that occurs when groups of riders ride together at the same speed. If all the horses are, for example, Walkers then very many of them will have at least some commonality in way of going. This can lead to the illusion of horses "standardizing" their gait.
It's very possible that a 17 year old horse is beginning to feel the pangs of "aging." I have three in this age range and all are a bit slower to warm up than they used to be. A vet consult may be in order to see if there are any signs of arthritis beginning to show up. Depending on the results of the consult use of something like Adequan or Cosequin might be in order to give the horse some "help" in dealing with minor joint pain. I note these two meds specifically because both have a good science behind them with professionally done clinical trials. A lot of the "over the shelf" stuff has all kinds of testimonials, but the plural of "anacdote" is not "data."
I've used Adequan on my 17 year old mare and for her it was a "wonder drug" that turned the clock back several years. She had undergone hock injections (her left rear does have some issues) but I wanted to try something less intrusive and with fewer possible side effects. I've known people who used Cosequin to good effect (one of our better local equine vets was a student at Auburn Vet. School when the Consequin tests were run and he endorses the product strongly).
These two meds are not cheap but they have a proven track record of effectivness.