I started to compare my runs by efficency ie output/input .This is speed in ~metre/minute divided by heart rate.
Eg a run that averages 10min/mile and 130bpm is 1760÷10= 176 meter per minute then divided by 130bpm 176÷130=1.35 ..obviously the higher the resultant number the more efficient the run .I got this from Joe Friels blog page.I found it a good way to compare the same routes at different efforts.
What shocked me was that I realised I am most efficient at 85-90bpm and 175-180bpm ..I know as you get older things should be more polar but wasn't expecting that ..so it looks like mostly walking and a little speed work from now on..interesting to see if that brings me on ..what are your thoughts