I say running because that's the sort of thing you do on a treadmill, but I didn't actually make it that far.
I went to my local independent sports shop, looking for a new pair of running shoes. I tried some on, walked around in them and when I found a pair, they offered for me to go on the treadmill they had.
Not only had I never been on a treadmill before, I'd never seen one in real life. But I've seen lots of people running on them in videos, so it must be easy. Turns out it wasn't.
I turned it on, it started, at a walking pace, and I struggled to walk. I constantly felt like I was going to fall off, even though I was right at the front (and the whole thing is surprisingly long). I had to hold on and could only really manage to walk properly for about two steps. There didn't seem to be much point in going up to a running speed.
When I got off I still felt like I was moving. It was unpleasant all round. Unexpectedly so. 0/10 never doing that again.