It is not uncommon when talking to a new acquaintance whom I just informed that I am a runner, to hear the questions “how fast do you run?”.
First off, it’s the wrong question to ask. If you are interested, you should say “what is your pace?” i.e. how many minutes does it take you to run a mile?
Secondly, as far as I’m concerned, the term “run fast” is an oxymoron. I’m more like a turtle slogging through the mud.
I’m more likely to answer the question with “I don’t run fast, I run half fast”. (be sure to say it quickly!)