This is a very dramatic piece. Did you already sight read the rest? You will realize that the beginning is actually not the hardest part of the piece. And I would just give you the general tip to find the absolute most difficult sections of a piece first. (And of course, difficult here means
difficult for you)
My quick guess would be bars 47-48 and 51-52, followed by the passionate section around bars 30-43 approximately. You will gain the technique to play the rest of the piece from mastering these bars.
And by the way check bernhard's reply #8 in this thread
https://www.pianostreet.com/smf/index.php?topic=10505.0He recommends looking at Heller Op. 45 No. 15 before tackling the Rachmaninov prelude. It has the same character and is much easier. (maybe after all, you are just looking for this character and not exactly the level of difficulty in Rachmaninov)