The inch is 3 grains of barley of course! At least the kind of barley they had In the 14th century. Alternatively the length of 12 poppy seeds in a line. You probably have lots of those lying around your house right?
The foot is about the size of a clown shoe, duh!
And the mile is about the length of 1000 paces from a malnourished roman soldier
It's a matter of habit. You intuitively know how much foot is, how much mile is, and it's only a matter of time and habit that you start intuitively getting meters and centimeters the same way.
But for as long as you make this transition, remember that:
1 inch is about 2,5 cm,
1 foot is about 1/3 of a meter
1 mile is about 1,5 kilometers.
Yes, those are not super accurate, but those are simple numbers and they give you a quick idea.