That's weird.
I have been using metric all my life until I moved to Canada. Picturing feet, inches, and miles is easy, once you figure out how long they are. Should be equally easy going the other way.
What's there to imagine, anyway? Once you know how long it is, that's all you need.
Maybe you're just special
But, I, for the life of me, can't imagine how long a kilometer is. I can imagine a mile, and I can use miles when talking distance, but kilometers escape me. The same goes with Celsius.
I'm a little better with the smaller measurements, like meters, and centimeters, but the large ones I just can't grasp in metric.