I too have a very hard keeper whošs 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 wonšt 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 hešs out on pasture full time (except at feeding time). We find hešs more lilely to clean up his grain if we donšt put the hay out until hešs 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 hešs pretty skinny. Needless to say, he doesnšt eat well at rides, so he needs to be constantly managed and coddled. (Išm the one you see standing in my horsešs pen at rides, hand feeding him grain, carrots, apples, mash -- whatever hešll eat :))
You just have to work with them.
You might want to increase your horsešs 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, Išd do what Roger suggests: consult your vet, and worm (rotating wormers) every sixty days.