Miles to K?

[magnuson]

If the Boat Race is 4.5 miles, I wonder what that would be in kilometers?
Reason for wondering is, hmm, maybe on the day of the Boat Race, we could have our own indoor boat-race-distance challenge?
Or maybe we should just drink pints and have fish and chips?

[jsho10]

that would be 7.242 kilometers. I'm game either direction, but I liked the sound of pints, fish & chips, and showing the race on the big screen.

Since when do we have to convert miles to kilometers?