And alas, I'm not Ennay (else I'd be training for marathons)
...
She explained it perfectly.
I just want to add a chart that I made for clients to take to the gym with them. It uses Perceived Exertion as a way to judge degree of difficulty - all this means is: how hard are you working on a scale of 1 - 10? The PE numbers also roughly correspond to the % of your max HR, so '5' would equal 50%, '8' would equal 80% etc.
It's set up for a treadmill with walking and jogging, but there isn't any reason why you couldn't use the same general idea for an elliptical or bike.
Cardio Intervals — 40 minutes
Minutes ----------
Phase ----------
Perceived Exertion
----- 5 ------------- Walk ------------------ 5
----- 4 ------------- Fast walk ------------- 6
----- 3 ------------- Jog -------------------- 7
----- 2 ------------- Fast walk ------------- 6
----- 2 ------------- Run ------------------- 8
----- 2 ------------- Fast walk ------------- 6
----- 2 ------------- Run ------------------- 8
----- 2 ------------- Fast walk ------------- 6
----- 2 ------------- Run ------------------- 8
----- 2 ------------- Fast walk ------------- 6
----- 2 ------------- Run ------------------- 8
----- 2 ------------- Fast walk ------------- 6
----- 2 ------------- Jog -------------------- 7
----- 3 ------------- Fast walk ------------- 6
----- 5 ------------- Walk ------------------ 5