A tip to getting through longer, painful songs, is never, ever use your overdrive. This will allow you to get drum fills often. this will:
1) give you a small rest
2) allow you to skip a few notes
3) you will have overdrive ready whenever you're in trouble
With songs like Run To The Hills, I used the drum fills as an opportunity to pause it and give my arms a rest, then went back to mashing the red note for another few mintues.
As a tip for "Dont Fear the Reaper", I played it probably 6-7 times, failing at the fast red notes and everything after that. To overcome this, I just made more and more runs at it until I went into those hard parts with full overdrive and a full crowd bar.