I imagine Americans who are well traveled within the contiguous part of the USA have as much mileage as Europeans who only travel in Europe. Both are roughly the same size. Europe is so small. I remember when I took a train from London to Paris, I thought "Wow, I see why Europeans are well traveled. That's all it takes to go to another country? I have to drive 8 hours to reach the Canadian border from my hometown."

Also, Hawaii technically isn't part of North America, so any American that has never been to another country but visited Hawaii technically has been to two continents. The distance between Miami and Anchorage is longer than the distance between Miami and London/Lisbon/Barcelona/Paris.