Should I still have a separate time to workout?

I go to summer school in the morning by bus but now I walk home, which from google maps is about 1.4 miles and takes about half an hour. There's tons of hills, and steep ones and I get pretty sweaty and it feels like a good workout but I don't want to sit around thinking that it's an exercise if it's not. Should I still be exercising separately at the gym, or is that enough? I'm 15, about 230lbs and trying to lose about 35lbs this summer, if that matters.