I would not recommend bonking in these fasted workouts. It is likely to slow your recovery and you now doubt slow way down when you bonk. Neither of these is desirable. 2-3 hours is a decent amount of time to do a fasted workout. Not sure you need to push longer unless you get to where 2 hours feels easy with no drop in energy.
The reason 2 hours on a treadmill takes more energy and makes you feel bonky than a 2 hour hike outside is that the treadmills is unrelentingly all uphill. No flats and no downhills. Also 30% grade is very steep and probably much steeper that you have found outside. A steep trail will be 15-20%. Most are in the range of 10-12%.