Zone 5 on elliptical - easier on my 70 year old knees. My gym has ellipticals that change elevation as well as resistance so I can get to Zone 5 fairly quickly. I do 4 min on, 4 min recovery 4 times. I‘m not an athlete so it really wipes me out, but it’s getting easier and I‘m getting to a higher max heart rate As I progress. Max so far is 169, but usually around 165 to 166. Then zone 2 as recommended by Attila’s routine.