Don't feel bad. You will gain strength and endurance with time. Look at me, I finally got up the nerve to raise my step height to 8" after year and years of only using 6". It was a good challenge, but one that I avoided because of my knee problems in the past when using an 8" step. Now, lo and behold, it's not my knees that are hurting, but my lower back. Has anyone else noticed that problem when increasing your step height? My lower back- which I injured at a part time job I had while in college ages ago- has been hurting me the past few days, and when I did another step video today with an 8" step, it was killing me. So I had to go back down to 6". I had no idea it was the change in step height that I used on Wednesday that was causing the pain until I did step again today. Very frustrating, but I have to listen to my body. I hate having pain in my back since I do a lot of my work standing up.