I’m 45 years old and trying to get in better shape. My son tore his acl last year and is trying to get back in shape for football season. I made him a promise that I would do this with him. The problem is I can only go about 30 seconds until my heart rate hits about 160 or 170 and I have to walk. Is this normal?? Should I just keep doing week 1 until I can go the full 60 seconds??