I have one horse with vision damage in one eye. He is riding well at the moment. I can't say how long this may last, as he could eventually go blind in both eyes. But for now, he is doing okay. It is an addition risk if he should hit a tree or spook and not see a tree. Many one-eyed horses do well though and adapt to their disability. Riding at night would be more dangerous as both horse and rider will be visually impaired. You are probably fine for endurance riding in the day, just have to be extra careful about roots and obstacles your horse might not see.
I would want to know why the horse lost the eye, as uveitis can jump eyes and you may end up with a horse that goes blind.