I pretty much agree with the previous posters.
My girl is 28 now and, right up until she partially tore a suspensory in September, I was riding her myself at least twice a week (some walk, a lot of trot, some canter, and some galloping), she was being used in one W/T/C lesson a week and one walk-only lesson.
The suspensory issue was not work related - she's blind, tried galloping downhill, voila.
She's 100% sound now so she's back to being used in one walk/trot lesson a week. I hope to return her to "real" work this May but we'll see!
Anyway, with her, I just watch for soreness post-ride and I make sure she doesn't get too tired. If she ends up sore, we do less the next ride.
The other thing with her is that I really really focus on getting her condition up, prior to doing anything strenuous. For instance, when I bring her back this spring, we'll start out just walking the trails (5 miles, hilly, etc). As that becomes less taxing for her, we'll start trotting on the uphill sections, keeping it at maybe 30 seconds of trotting every 5 minutes. Then, we'll start trotting for longer stretches since trotting builds muscle. As she acclimatizes to that, we'll add a short canter here and there. Then longer canters to build wind, etc. Only after none of that is really hard for her, we'll start galloping.
The primary gait will stay a walk and after every long trotting/cantering stretch, she'll get a good 5 minutes to walk on a loose rein and work it out.
Often what I would do is have one ride a week that was really intensive with lots of trotting+cantering, then the other ride that week would be a "walking exploration" ride. We'd go explore some trail we had never been on before and walk it, miles of walking. We might have a short trot/canter at the end since Lacey loves her running time, but the goal of that ride would be to get her mind working. The goal of the other ride was fitness.
Some horses aren't capable of hitting that^ sort of level in their twenties, so that's something to keep in mind. But if you're mindful of her joint health and she's sound now (and then stays sound), I don't see trying to see how fit she can get as a problem.
It's been proven that movement is great for arthritis so go get that girl moving! :)
ETA: the other thing I think is important about the level of fitness I got Lacey to is that I never asked her to do anything really strenous two days in a row. We'd have our fast fitness ride on Monday, usually, then our meander ride on Wednesday, then she would have her W/T/C lesson on Friday, and her walk lesson on Sunday. That way her body had a chance to somewhat recover+perk back up post-ride.
I found that even just walking her over hilly terrain for 3 hours everyday really took it out of her (we used to lead trail rides at a summer camp). Whenever she would end up with a lighter schedule, like 1 hour of walking Friday morning, then a break over the rest of Friday, she was perky for a couple hour long walking ride on Saturday morning. But if I asked her to go all day on Friday, then go for a couple more hours on Saturday, she would...but it would be obvious that her heart was not in the ride.