Actually - I would suggest finding someone who has a horse with a very **non** competitive attitude. One that is very laid-back, unflappable and easy-going that can baby-sit yours.
If you put two competitive (or anxious) horses together, they will feed off each other and just end up being 4x as worse together as they are alone.
Make sure the rider is OK with this, too. There are some folks I won't ride with because, even though the horse isn't competitive, the rider is not tuned in, and will ride past me, oblivious to the hissy fit going on until I bring it to their attention.
Is there anyone doing a SE ride (yellowhammer, hahira, carolina) LD who has a horse like this who would want to ride with me? My horse is like this and it has kept me from endurance. I did a 30 on him a while back and recovery wise he was fine but I was too nervous to enjoy myself much. I've been conditioning him so I know physically he can do it.
If you want to just ride in the back at a slowish pace just to finish and would like some company please let me know.