Bernstein had been playing Rhapsody in Blue since he bought the sheet music while learning piano in 1932, 44 years before that performance.

There's a quote of his in wikipedia

Rhapsody in Blue is not a real composition in the sense that whatever happens in it must seem inevitable, or even pretty inevitable. You can cut out parts of it without affecting the whole in any way except to make it shorter. You can remove any of these stuck-together sections and the piece still goes on as bravely as before. You can even interchange these sections with one another and no harm done. You can make cuts within a section, or add new cadenzas, or play it with any combination of instruments or on the piano alone; it can be a five-minute piece or a six-minute piece or a twelve-minute piece. And in fact all these things are being done to it every day. It's still the Rhapsody in Blue.

I have it on on an LP of the LSO conducted by André Previn, which I bought 40 or 50 years ago. (Seiji Ozawa conducting the Sesame Street orchestra reminds me of André Previn.)