I too have a very hard keeper who�s also a picky eater -- a bad combination. He gets six pounds of 12% sweet feed twice a day. He will usually eat that, but if we try to feed him more, he refuses it, and sometimes won�t even finish his usual ration. We also find that if we add ANYTHING at all -- even corn oil -- he refuses to eat. He refuses to touch alfalfa cubes. But he gets as much good quality hay (timothy with a little alfalfa mix) as he will eat (usually one-two flakes twice a day) plus he�s out on pasture full time (except at feeding time). We find he�s more lilely to clean up his grain if we don�t put the hay out until he�s finished, and if we leave him strictly alone while eating (no cleaning him or his stall during meals).
He maintains fairly well on that schedule -- but by the end of the season he�s pretty skinny. Needless to say, he doesn�t eat well at rides, so he needs to be constantly managed and coddled. (I�m the one you see standing in my horse�s pen at rides, hand feeding him grain, carrots, apples, mash -- whatever he�ll eat :))
You just have to work with them.
You might want to increase your horse�s work gradually, rather than all at once. fifty-two hard miles sounds like a lot just two- three weeks into a conditioning program.
But before panicking, I�d do what Roger suggests: consult your vet, and worm (rotating wormers) every sixty days.