Originally Posted by
Jim from Boston
Not to sound like a hardcore roadie, but for me fitness is a major reason to cycle commute, and I consider my situation not “close,” but “optimal.”
This was not a conscious decision, but resulted from those “other things.”
I am occasionally asked by suburban colleagues who live closer to work, why I don’t move out there from downtown. My reply is that for cycling purposes, the distance and routes are perfect, meanwhile thinking to myself,
“Why do you think they call it ‘sub-urban’ living (like ‘sub-human’)”? 
I definitely can't hit 12 miles in 40 minutes when commuting. It takes me about 42 minutes of riding to do 11, then add on time for traffic lights and such. I usually average around 14 mph, sometimes faster sometimes slower. I'm also hardly ever trying to push myself. Loaded down with clothes, shoes and lunch, my commuter is probably over 15 lbs heavier than my road bike, which I have no problem keeping around an 18 mph avg.
I think around 8 miles sounds about perfect. Long enough where I still get over an hour of cardio in the day, but short enough where I don't feel burnt out after a few days in a row