I think this is where I got to - it was interesting taking an HR monitor out a few times, so I can see and quantify what's going on, but it did pretty much match my perceived effort. It's useful to have a number to tell me to ease off a bit though, I tend to go near the top of zone 3 while I've got the energy to do so.
As a rule of thumb on a turbo or if you don’t have one a gym bike do a 20 min warm up then go as hard as you can for 5 mins in the end of which you should hit near your max.
On waking up before you do anything take your pulse rate twice over a 30 second period obviously double to give minimimum.(this is useful to take monthly if it goes up a lot you are overtraining)
Given your max and min you can calculate your threshold (see google)
This is where your body begins producing lactic acid .
All off the training routines on the nett are designed to take increase your lactate threshold and therefore speed and distance.
None of this is exact but we ain’t Olympians just trying to improve its better than not trying at all cos it ain’t perfect