How to start working out?

I might've asked this question before, but whatever. Here's what we're working with:

- 5'9" and 140 lbs
- couldn't do a pushup if my life depended on it
- couldn't run a hundred yards either
- I can swim 60-70 25-meter laps at the swimming pool though

Whenever I try to start working out, everyone's like "oh, just start with ten pushups every morning. Easy". Hell no it ain't easy. I do go swimming two to three times a week, but any other form of exercise just kills me.
How to start working out?
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