I understand--I was like you. I look really fit, but have always had poor cardio health (never played sports that required running).
I started the "couch potato to 5K" program from www.coolrunning.com and it was a great way to work your way to a 5K. I've slacked off a bit since then, but I can still run a few miles pretty effortlessly, which I SWORE I would never be able to do.
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