Yes, conditioning! It's very similar to conditioning a human to run a long distance. You have to mix distance days with speed work days. You have to condition using hills....and of course, you need days off as well to recover.
...and I agree about CTRs! Especially since they take into account the condition your horse is in at the finish, so people are discouraged from all out killing their horses...and they are shorter...so a good place to start.
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.