Multi stage / Multi Days cycling

I have not recorded such long trips, but I have done multi segment trips.  At the end of a ride, you can press the START/STOP button to stop recording, then don't SAVE or discard the ride, but just power down the 840.  When it boots up the next time, you can hit START and continue recording.  This will created one activity rather the separate activities.  This is all good as long as nothing goes wrong and you lose the whole thing.  Personally, I'd rather save each ride.  I know there are also apps/sites where you can post process rides either dividing one long one into several shorter ones, or concatenating several rides into one long one.

Power down or sleep?

Power down.  I even do that for longer stops.  I don't use Sleep as it still drains the battery, though slowly.  The only reason to use sleep is not having to wait through the reboot if you powered down.  You can even power down without hitting pause and it'll ask if you want to continue when you power it up.  Try it out on a shorter ride to see how it works and verify it'll do what you want.

I know in the past with previous units there were issues with really long activities, but I think that's no longer a problem with the 840.

I guess I should mention that I'm referring to saving the activity, not navigation.  Navigation is totally independent of recording.  You can start and stop either without affecting the other.   I strongly recommend creating separate courses for each day rather than one long one.  IDK how well the 840 handles really long courses.   On previous units I used to limit course length to <100km.  

Thank you for the info. 540 here btw.