I think the simplest measure of working it too hard is if it won't hold its rpm. If your cooling system is in good shape and you have good oil in it etc then loading it sometimes until it will just barely hold its rpm won't hurt anything, that's what it was designed to do. Oil and temp gauges should remain in the 'reasonable' range if the cooling system meets spec.
My experience has been I've had to shift down a gear to mow back up a gentle slope that I could easily mow in the downhill direction. Just climbing the slope loaded the rig - with 4 ft mower - too hard if I was cutting tall wet grass. I doubt I could mow that stuff in either direction taking a full cut with a 5 ft mower.
Since you already have the 5 ft mower - I would take less than a 5 ft new cut, overlap the previous cut, wherever there is heavy growth. You'll find a balance with what it is capable of.