Is it bad to exercise every day?

For the core, I would suggest planks and side planks instead, done 2 or 3 times a week. As for the treadmill work, 20 minutes a day is fine unless you start doing high intensity interval training within those 20 minutes.

 

How do you feel? Are you tired all the time? Or are you invigorated by the workout? I don't see how 20 minutes a day is too much, but your body will tell you if it is.

 

For want your doing everyday is fine. When I was working out it was every other day and, each day was a different area, leg's one day, arms one day, chest/core the next. So I had a day of rest for my body and each zone had a few days to recover but on the same note I would work out for 2/2.5 hours at a time. I will add even on my "off days" I would still get some cardio in.