2,000 vertical metres was the was the greatest single, uninterrupted climb we had to do in the Himalayas (at altitude too!), and that's a pretty big day out with full luggage - I wouldn't have liked to do much more than that.
Rather than 'spinning' however, which completely kills me in about 15mins, I found the best approach to long climbs is to choose a slightly higher gear and 'plod' - turn the cranks at about walking pace. We found we could keep that up for a whole day, no sweat, even chatting along the way. You have to put more weight on the pedals but it's not nearly as intensely aerobic. AND... it's reassuring to know you've got a couple of granny gears left for technical or *very* steep sections.