TMonk pretty much nailed it.
Only thing I'd say different is that on very long climbs, I stand for 30 seconds or so, periodicallly, just to recruit different muscle groups, and mix it up mentally.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.