I grew up using feet, inches, pounds and ounces. They tried to give us the metric system but there was so much resistance, it was abandoned after the first year's efforts. I knew that a meter was a bit more than a yard, and a kilo... well... as a child of the 60's we all knew what a kilo weighed...
Then I moved to Asia, where it's ALL metric. In Japan, meats are sold in 100gm units. Distances and speeds are all kilometers. And surprisingly, I had no trouble switching my thinking. These days I think metrically. Even scuba diving... I figure my depth in meters and my tank pressures in bars. But... Altitude... I simply can not think of mountains in anything other than feet! No trouble with my physical height in meters or centimeters, but mountains? Gotta be in feet! Go figure!