100 push-ups in a sitting isn't easy, doing all 3 together every day isn't easy. Most people couldn't do 100 push-ups and 100 sit-ups but the main hardship is doing it every single day without a rest day. No beginner could do that, which is why it gave Saitama super powers. I should add even when I was running a lot every day I couldn't do 100 pushups in one go, maybe over several sessions in a day but doing 100/100/100 squats/situps/pressups is pretty impressive.
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