Well it can be hard for an out of shape horse to properly use himself no matter what you are trying to acomplish...and as for being out of shape this goes for a to fat or to skinny horse or one that is just undermuscled.
You need to build up his muscles so he can use them properly. Try hill work for his hind end. Lots of long low collected trotting to help his topline and do alot of flexation to help his neck. Once you have him built up a bit I bet he tracks better.
Also as for a XC course, which I am by no means an expert an this is just a personal opinion, perhaps map out a short section of trails and ride it at a fast pace to help his stamina. Then you can add jumps an hazards to it to form a XC course. This way you build your horse up to it slowly. You would be surprised how similar a XC course is to a trail...its just done at a higher speed with more obstacles.
But im by no means an eventing person...I just know how to help build a horse up muscle wise...these are just ideas...they might be totally whack to someone who has done actual eventing.