Saw a guy hit 60 dials once and book nothing, so 40 sounds about right for a rough day.

Yeah watching someone do it at scale really changes your brain about it. For me it was my aunt batch cooking tamales every Sunday, she'd knock out like 60 in a few hours and freeze them, and I just started copying that with regular stuff. I cook a big pot of beans and rice on Sunday, portion it out, and I'm set for days without thinking about it. It's really just doing the work once so you don't have to do it five times.

400 yards in a month is a lot but honestly your knees hurting might just mean you need better pads or a different stance, not that you should slow down. Carpet guys who stretch it tight and take their time end up with way fewer callbacks than the ones babying their joints. Push through it, your body adapts.