To be fair, root comment only says "mice will run 10k a day" but it isn't 100% clear what that 10K actually refers to, could be steps, could be meters.

In the running context, a 10K means 10 kilometres.

That's awfully European of you, what if the mice are from the US?

US runners also typically train for kilometer races at distances shorter than half marathon.

I've even seen a few 50Ks.

Means the same thing here.

“I ran a 3.1mi this morning and I feel awful!”

No, distance events are still referred in kilometers in the US.

My physics teacher always assumed it was potatoes when the SI-unit was missing