ME workout progression

I’m training with the 16 week mountaineering plan. I have never done muscular endurance training before, so for my first one, I started conservatively and did 20min warm up to the top of Z2, followed by 40min on the stairmaster, carrying a backpack with about 12% BW. During the workout, my HR varied between 135bpm and 150bpm. My AeT is 135bpm and AnT is 155bpm. I did not feel HR to be the limiter, but I neither felt any DOMS, or more tired than usual, the day after the workout. So I am wondering if I should increase the weight carried, the duration, the speed of the machine.