@goosehd
Agree with Nik.
Lactate lab is best.
Short of that, establish your max heart rate (warm up, run uphill at max sustainable speed for 10 minutes, rest 5, do it again and take max heart rate of second run --- many other ways available). Subtract your resting heart rate from max and divide range in 5 zones. Add them to resting heart rate.
Remember that zone 2 is volume training. In order to see benefits (MAF test or just faster at same HR) takes at least 5 hours per week running in zone 2 imho. You can also use running at conversational pace, nose breathing etc. Don't be too dogmatic about it. When running uphill and you pop out of your zone it's fine as long as your HR quickly returns back to zone 2 after.
If you got back from a 70min z2 run and felt you could do it again you are perfect. That is how it is supposed to feel.