Heather, WTG on day 1 Feels pretty good doesn't it?

Your added walking and yoga shouldn't be a problem - the excessive/extra refers more to the intense kind of cardio that is the 20 MAS. Because this kind of cardio is more anaerobic than aerobic doing too much of it can lead to muscle loss (Increasing protein can help prevent this). The type of low/moderate intensity you'll get from walking/yoga will not pose this same problem.

HTH
Nat