I distinguish in my mind between riding for exercise/ fitness, schooling, training, and problem solving. Obviously these blend somewhat, but they are also quite distinct.
For instance, I ride my coach’s school master mare both fitness, on trails, and schooling. For trails we just hack out. If she spooks or gets heavy on the forehand obviously I would correct. But we aren’t working through her repertoire and any good solid rider could do this.
I also do sessions where I school her in the ring, and we work through her existing repertoire. Here I need to be careful to keep my aids and riding, and her performance, correct. But I am not training her anything. This requires more skill than hacking her out.
On my own horse I do training, as well as fitness trail rides and schooling what we already know. I actually teach her moves she didn’t previously know existed. This is very different from schooling moves the horse already knows and is confident in. For instance, I was continuing to introduce travers at the walk yesterday.
Finally, I mention problem solving. This is when the horse has an ongoing behavior issue that you need to sort out through training. Maybe the horse rushes, spooks, or balks on the trails. Maybe like my horse there is a wierd combination of intermittent ring sour/ over excited in the ring. This has taken some creative problem solving. This is also the level where riding especially a strange horse can be dangerous.
Obviously you might need to school on a hack if the horse decides to go inverted or blow through your aids.
But I think for that price you are not going to be teaching him new things.
And it is up to you if your exercise rides deal with problem solving. I would think the combination well schooled horse and high level rider shouldn’t present too many difficulties. And if you do find you are riding a nutcase you can always drop that client.
Go slow, do a trial ride with owner present, ask the questions you would if it were a lease, and trust your gut.
It’s true that some riders might hire you hoping to get problem solving rides on a sour horse or training miles on a green horse, for half the price of a trainer. You will need to figure out how to sniff those out and decline, or have a sliding scale where you charge more.