Hey man, yeah that was one of my main takeaways from the program- if you really want, you can absolutely destroy yourself (in a good way). You have to be honest with yourself and how hard you’re working though.
I only did weighted walking lunges as assistance during leg days because that’s all I know how to do. I didn’t feel like learning new movements or working on mobility (although next time I do the program I may do that). On backsquat day, I think, I’d hold plates in my hands, hanging down at my sides. I’d take strides that were slightly longer than what I’d do when walking, and lower myself down under control every single time. Barely touch my knee against the floor and immediately push off. If your knee slams down or you’re moving too quickly that’s begging for an injury. Always slow and controlled, and speed will come with time.
Zercher/front squat day, the lunges were normally done bodyweight, and just kept going until I couldn’t anymore. Then do another few sets.
I didn’t have any real planned progression for them tbh. If I hit big squat numbers I knew my legs would be toast, so I didn’t expect too much from them. Zercher day was “easy” so I’d do as much as I possibly could to burn out my quads with the lunges. I’d pretty much go to failure on every exercise every day and I recovered decently despite eating at maintenance. If I remember correctly it’s 3days/week so I feel like that’s why I could get away with it… and, especially at the beginning, the weight wasn’t too heavy.
I’d say pick a number of reps that you think would be achievable for 2-3 sets, and by set 4/5 is pretty challenging or you fail. Work up to that, and once you can do that consistently, add weight and start all over.
So for the last leg day you did-
Walking lunge BW+40lb, 5x12 for two weeks, then add 10-20 lb and work up from 5 sets of 7-8 to 5 sets of 12 etc.
Edit: and this is just what I would do. There are other people here on the forum that may have different and much more informed advice than I. I’d tag a couple people to have come in once and a while and ask them some of your questions and for advice.