Just did week 5, day 1 of the C25K program, and it was TOUGH! My core is very stiff today so that made it all the more difficult to complete the workout. Not to mention the fact that you run a total of 18 minutes in this workout and only walk 14. I can feel some wear and tear on my knees and ankles so I am reworking my cardio plans for the next week to take it easy on them. However, I fully intend to do week 5, day 2 on Thursday (the day before I leave on vacation).