Quadruple-Digit Post Count
Any source for that? (study, huge amount of anecdotal evidence, etc.)About 18kg (40lbs) is the upper limit if you want to keep your knees healthy
GoRuck challenges don't go above +30lbs/13,6Kg.
On my usual trail there's a steep hill which gets my HR up to MAF and even beyond when I keep up my pace, but it's only like ~10% of the total distance of the round, so you can't constantly use that.The best way to get your heart rate in the correct zone is to add elevation gain of some sort, be it a hill, stairs, inclined treadmill or a box step.
Inclined treadmill is good for the constant elevation, but I don't see the point of walking on a maschine. Part of runnning, walking and rucking is to do it outside and enjoy nature.
Btw @damogari rollerblading might be a good alternative. Faster than running, but not as expensive as cycling.