Dear God, I think I'm going to die

The only exercise I've done in years is speed walking through the grocery store. Brilliant me decides it's time to start getting in shape, so what do I start out with?
The "Boxing Shuffle"- set to "Pump up the Volume". No warm up, no stretching.
After 2 minutes 44 seconds, I couldn't do any more.

I used to take Kempo in my teens. I remember very little except the punches and blocks ("throw your elbow back! Bend your knees!!")... and of course, the boxing shuffle. That's what my instructor called it anyhow. All it is is rhythmic shuffling from one foot to the other, throwing punches or bocks with each change. Anytime we got "in trouble" we had to do that dreaded shuffle- sometimes for the entire session.
And now I can't even last 3 minutes

I'm humbled enough to admit that baby steps seem to be required. But it has to stay interesting and I want to "feel" that I had done something the day after.
I'm naturally very flexible so stretching feels like nothing at all.

Suggestions?