In playing Op. 23 No. 5, particularly in the alla marcia parts, touch control is very important and plays some role in avoiding fatigue in the arms. There has to be eveness but fluidity too in the playing. Rachmaninoff employs a number of touches, and that variety demands different uses of the playing mechanism, rather than just one. Responding properly to the variety of touches helps to avoid arm pain too. So it's important to notice whether a passage is legato, nonlegato, or staccato. Speaking of legato, be sure not to catch the middle note of those three-not slurs in the pedal. And some sections require no pedal at all, such as measures 8, 15, 16, and certainly the coda.
Distinguish also forearm octaves from wrist octaves (or chords). On page two, for example, the first 8th notes on the downbeats in several measures are accented and are forearm octaves played with weight for tone production. The following chords in the octave mold are staccato and played with the wrist, are intended to be quieter, and are background only. The last four 16th octaves in those measures are scalar, melodic, and legato. Be aware: If you play background music with equal force to foreground music, you'll tire quickly and never get through this piece.
On page 2 in the 2nd, 3rd and 4th measures there, the RH octive scales played legato need to be fingered 4-5-4-5 in the top notes. It not only connects the notes better, but allows you to play those figures with the fingers and less with the wrists, which also reduces arm fatigue. In the very last measure of page 2, be sure to brush the repeated notes in the RH with 3-2-1. That too serves to keep the wrist more quiet and to execute in a more relaxed way. (In this piece you need to find every opportunity to relax that you can!)
After all of the bombast of the last page, some people rush like a house afire to accommodate the coda marked leggiero, a totally different touch from what has just occurred previously. (Beethoven used to love to pull these pranks on the nervous system too.) And, of course, performers' arms are so tired that the short cadenza gets messed up as well. Turn on the metronome and discover how it is really to be played--far more unrushed than imagined--to be consistent with the rest of the piece. Also you need to totally reconceive the RH note groupings there, as they are not apparent. For instance, in the first measure of the coda, despite how the score appears to the eye, the last two 16ths are mentally paired with the first two 16ths in the next measure. Similarly, in the second measure there, the 6th, 7th, 8th, and 9th 16th notes form another easily-executed four-note grouping, despite appearances of the written notation. Playing it that way also reduces physical effort. Several authors have pointed out that note groupings that you see on a page sometimes need to be recast to better choreograph the hand and make the passage more playable. You're not changing any notes or rhythm really, simply reordering the note groupings mentally which enables the hands to better cope technically. This is one of those cases.
Finally, pay attention to dynamics. True, there are many parts marked f and ff that consume energy, but there are also fallbacks to p and other diminuendos too. Take advantage of every one of those changes to put less stress on the playing mechanism. Also the quiet, lyrical middle section enables you to regroup and gather your forces for the last two pages that revert to Tempo I.
So there are numerous tricks that you can employ to take the strain off yourself in playing this piece evenly yet with fluididity. Having said all of that, it's still very demanding nonetheless and does require stamina! No two ways about it. I agree with martin and tosca1. Do not play through pain either in the forearms or on the backs of the hands. Get up from the bench, stand straight, drop your arms and dangle them like a couple of loose ropes for a while. Then wait awhile before resuming practice on that part of the piece. Work instead on the middle lyrical section. Hope this helps.