I have never been a proponent of the often standard five or six day a week arena schooling sessions when at home that many commercial stables endorse. The only times our horses are ridden for five consecutive days would be when they are ready to start showing at rated competitions. They make the transition easily.
Meanwhile, at home, some horses may be ridden two days on, then have a day off, and the work the next two days.
Other horses have made the quickest progress with only three rides per week and a hack down the road.
It is up to the trainer to quickly determine what schedule keeps each horse happy, thereby maximizing the horses' willingness to learn, retain, and apply new skills consistently.
So, presuming the horse has had a day off after day 2, day 3 will begin with the same lessons as in day 2. If all goes as well, or better, than day 2, I'll introduce two variants:
First, changing directions across the diagonal, with, depending on the horses' best inclination, either a downward transition from the trot to the walk at the center, or a downward transition from the walk to the halt.
Secondly, if I'm getting a nicely balanced trot through the 20 meter circle, I'll utilize the point at which I touch back to the rail to ask for the upward transition to the canter, and circle once or twice at the canter before continuing the downward transitions through to the trot, walk and halt.
Throughout every ride I am asking the OTTB to do somethings that they are often unfamiliar with: To keep thinking and listening every stride while being willing to
improve their suppleness and quality of movement throughout.
Some OTTBs' have left the track as quite ratable rides, others not so much.
Prior to introducing a hack down the road, it's essential to have a horse that is listening to the rider in the arena.