I prefer avg. HR, since HR can change depending on terrain, body condition, weather, etc. With avg. HR also don't need to keep monitoring, making the run feel more natural. Run by feel and slow down when necessary.
Check avg. HR during junctions or midway into the run.
Better off running a high 4 min/km last km which would help your long distance running a lot more. Unless you were doing strides after the run (after you take a short break), then that pace is good since it trains your stride extension. Other than that, I seldom go 3 min/km even for my pace. I do strides after easy runs after taking a break. Able to run 2.50 min/km for the 100m x 6. But this is never really done in my threshold/long runs since an extended surge (1-2km) cuts time more than short bursting (100-200m).
If you wear carbon-plate shoes, it will naturally make you run faster since it promotes a forefoot gait. That's why I prefer to run slow in trainers. Apart from this, it takes practice to run slow (though not as hard as run fast). Just reduce your stride length and cadence, can try practising this by doing slow jogs at home. Don't alter foot strike to rear since it causes higher risk of injury. You'll eventually get the hang of it.