I agree with the other respondents- especially with riding across the long diagonal. And remember, too many turns aren’t so great for horse’s legs. If the ground outside is too hard for trot/canter work, what about hill work? If you can trail ride at a real working walk for a longer time, you can build some endurance. What about hauling to another arena or trail on weekends? Maybe you could ask the barn owner to roto-till a track around some outdoor paddocks, or in a pasture, to soften the dirt.