I've just finished 26 days on the Master-Cleanse and I kept up with my regular gym routine. Lifted weights and ran 2 to 3 miles a day (3 times a week.) I've also hiked 5 to 7 miles 3 times a week. I did get lightheaded once but that was because I was seriously dehydrated. If you drink lots of juice you should be OK but let your body tell you how much is enough.