Do I have to go to the gym if I'm doing cardio and situps around the park?

I've been doing a lot of cardio lately along with situps, and I'm doing quite well. But some of my friends are saying that i should go to the gym to workout. Do you think it's necessary for me to go to gym if I'm already doing cardio outside of the gym? I run 15 times around the park everyday.
Do I have to go to the gym if I'm doing cardio and situps around the park?
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