I've never been a runner, but, recently and at nearly 50 years of age, I decided to give it a go.
I couldn't run the distance of half a block when I started. But I kept pushing myself until I could run the block, then two, then three, etc. At first, my lungs felt like they would explode! And my knees felt like the tendons were shredded. My back took quite a pounding. I feared I was doing more harm than good! Yet, my body did get stronger, and began to adapt to the new routine. And I want to just keep running!
I don't know that I will ever run even a mile, or anywhere near a 5k or half-marathon. I will keep running, and just see where it leads. It's been an interesting experience! (I was inspired by the Biggest Loser contestants who run whole marathons! If they can do that, what's my excuse?!)