to which I have learned to reply "as many as I can" with a goofy grin. If the questioner persists or asks how many miles I shrug and say "what difference can it possibly make?"
Really, I will always be riding more than some people and less than others, and I do it for myself not for someone I might never meet again. Plus I would rather log 20 miles of good hard effort, smooth quality and safe riding than 40 miles of mindless noddling along.
The people who know me and know that I ride are aware of about how much I ride, and really it's not anyone's business but mine.
If worse comes to worse, I excuse myself and go urgently looking for something in some other part of the store, or if out on a ride, claim I can feel my muscles getting stiff and need to start riding again right now.
marni