dear sir general sir whatever!
i had a very similar experience with a interesting difference: it was playing guitar, my former instrument. well, let me tell you what happened.
i did study, in my undergraduate years, castelnuovo-tedesco's sonata for guitar, a nightmarish piece of this instrument repertory.
then, when it was cool enough to me and, after playing its separate movements in students' colective recitals, i decided to propose it in a full-lenght recital, together with some other works of the composer, previously studied. well, that was the beggining of such an amazing experience...
the recital was very fine until its
grand finale: the sonata's mighty fourth movement. oh boy...what a mess. after almost an hour of music, both my hand failed in the fast figurations and arpeggios and the result was pure crap. lots of flaws, very little strength, very very little speed...in other words, the
bravura turned into a
tenerezza!

shortly after, i travelled to have some classes and - as it wasn't enough - my teacher occupied three and a half hours of classes pointing out problems and proposing solutions to some pitfalls in my approach with that. it was so frustrating that i called a day, and abandon the work.
after sometime, i decided to resume the study, but i thought that i should improve my technical means to render it as i wish to. so, i began studying the scales, right hand
arpeggi, tricky passages. well, anything that could be the slightest problem together with a bunch of standard technique related to that. it took about an hour a day, for almost a month, until i ended up playing those exercises at virtuoso level. but the results were not in the sonata itself, especially because my playing was keeping mindless.
i did stop it for some weeks, and then, finally then, the results were amazing. nothing was a problem anymore, i could play it far above my planned
tempo, flawless or almost. in the next recital, i scheduled the work and it was very cool.
to me, it's undeniable that exercise, practice and time span were decisive in the solution of this work. when you say that there was not a direct transfer of ability from the studies to the "real works", i think that's quite natural. btw, now in my piano studies i apply this directly: what is complex, or "difficult", in my very first sight-readings, i develop exercises and a plan of attack to solve. each new work it comes more naturally and faster. so, that's a very good way to me to work.
well, about bernhard. he's a great person, with a great heart. i feel very lucky to did find his writings up here, because many of them made me think and develop my ways of both studying and teaching.
did i went too much off-topic?

hope it is of any help.
best!