I agree it's not just mileage. Or just time for that matter. They are other factors too.
I used to do 15 miles each way from central to North West London, that took over the hour and really wasn't particularly pleasant. No showers at work made matters worse. Sometimes I'd work late and come the time I left, I didn't always fancy riding home. The tube station was across the road and I could be home in under the hour. So, yes, I left the bike at work a couple of times. I'd cycle maybe 2 or 3 times a week. Tube or motorcycle the other days.
So to distance and time, I'd definitely add 'alternatives'!