I'm as mystified as you about my speedy (actually instant) recovery from major surgery (albeit outpatient). A good friend of mine had the identical procedure two months before me, and even after 8 weeks he was still feeling disabled but was kind enough to give me very numerous tips--and as it turned out, none of which I needed. As soon as I left the hospital and thereafter, I felt no pain (I mean zero). To placate the doctor, I had to lay low for 48 hours and impatiently complied with that rule, but the very next day I was back to full routine and activities, again without even a twinge of pain. It's something I'll never understand or be able to explain, although I'm truly grateful for my good fortune. I do know this though: It's well established that on the Pain Scale of 0 to 10, 10 being the worst, that one person's 10 might be another person's 1, as it's all subjective. Anyway, I'm very glad to be back at PS so soon!
On Rachmaninoff, I believe that most of his scores look easy until you start to study them. Unlike Bach, Haydn, Beethoven, Chopin, Brahms or the others, Rachmaninoff, except for the very early opus numbers, didn't compose works for other pianists. In his day he was a composer, touring artist, and conductor. When it came to piano music, he wrote it all for himself, as he was not bashful about playing much of it in his recitals, which spurred sheet music sales royalties, of course. So in writing for himself, he didn't spare the difficulties, being one of the best virtuosos of his day, the other being Hofmann with probably Moiseiwitsch in third place.
I've played a good deal of Rachmaninoff. So if I just think of a few of the preludes, in Op. 23 for example: In No. 1, because of constant neighboring and passing tones, every 8th note has to be pedaled throughout the piece. Sounds easy until you right leg drops off! Keeping track of the ever shifting accidentals is a major chore. Therein he employs four levels of writing: the RH melody, duets in the bass, background accompaniments, and cross-overs. Or in Prelude No. 4, the melody becomes immersed in filligree, yet has to sound with clarity. This piece also is almost an etude in sustained legato. It is also built upon polyrhythms. In some places there is cohabitation of the hands. Melodic chords and double note passages have to be carefully voiced. And the piece has one of the most powerful crescendos to a climax that has ever been written, requiring artful timing and dynamic control to bring it off just right. If that "point", as Rachmaninoff calls it, isn't handled well, then the rest of the piece is as good as destroyed. Then take the famous No. 5. Here everything is a study in contrasts: Distinguishing legato from nonlegato, foreground from background, wrist octaves from forearm octaves, playing top and middle melodic lines in the very legato lyrical section. It takes a lot of artistic ability to do that convincingly. And then I think of No. 10, seemingly at first an innocuous trifle. There you have to make the LH sing, bring out a duet between the hands, etch a recurring sigh motif, voice melodic chords, etc. And in the section prior to the coda there are humongous rolls that were probably difficult even for Rachmaninoff with his large hands. To complicate it further, many of the professional pianists drop that section below tempo to leisurely take the rolls. Problem is that the score is not written that way. The rolls are dangerous (meaning possibility of incurring injury to the hand), so I practiced them no more than 10 minutes at a sitting, and when I recorded it, I played the rolls up to proper tempo. That was hard to do! And I could go on and on, but you get the idea.
I should also point out that in many of the preludes in both volumes, Rachmaninoff wrote short cadenzas that are very intricate, require exhaustive practicing, and and are sometimes killers to play accurately and effectively. Take the one ending the coda of the "Melodie" I posted here. Rachmaninoff made two recordings of this revised piece. The first one almost sounds like a mechanical practice tape probably never intended for release. There he battles through the cadenza. Then he also has a very artful recording where he did it most beautifully. Then there is Berezovsky's video. He plays it in a very large hall as an encore using the sheet music. When he reaches the cadenza, he omits it completely and advances to the last measure of the piece to end it! (I shoud say that have great respect for Berezovsky.) Given that, I probably deserve a bit of credit just for playing that cadenza for better or worse!
Volodos with his incredible technique probably does the best job with the cadenza, but how many of us have Volodos' technique and can play like him? As I see it, the difficulties in Rachmaninoff's scores that demand nothing less than musicianship and artistic piano playing are for real. I also find that this composer had a knack, not so much in making the easy sound difficult, but rather for making the difficult sound ever so easy in performance when played by one of the greats!