Bodyweight makes a huge difference for me, particularly after the first mile or so. Extra 10 lbs from doing squats and DL would increase my times by ~30 sec/mile. I've been running with a HR monitor for 20+ years and the data has been pretty consistent although pace has slowed. Sustainable pace went up from ~5:30 min/mile to probably >6:30 over the years over a 2-4 miles training run, but steady state HR has consistently been around 167 bpm. At race pace close to anaerobic threshold I'm around 175 bpm and that's been consistent for more than 20 years. My max HR in the 90's at age 21 pushing myself close to passing out, I could briefly hit 201 bpm. I used to run at least 2 - 3 times/week until 2 years ago and routinely maxed out @ 193 to 198 bpm at the end of my runs. As recently as Jan this year at age 44 I maxed 193 bpm at the end of 2 miles at race pace.