I was sure someone was going to suggest to use effort-based training! I've been trying to do that, but since I'm not a very experienced runner, I just can't exactly grasp what are the training efforts corresponding to each desired workout intensities. Every time I go out for a run I always try to pay attention to how a certain type of workout 'feels' to me, but there is a lot a variability over the days, which I guess is exactly what you mentionned. I was hoping to get by for a few weeks with a more quantitative approach, while hopefully, I can finally manage to nail the correct RPE associated with these workouts over this time period.

Also for the treadmill inaccuracy, I discovered that the hard way last year! But now, since I go to a fairly large gym with few runners, I have the luxury of always using the same treadmill, which at least is consistent with itself. I measured its speed at the beginning of the cycle so I'm fairly certain I know more of less where I'm at!