Somewhere in between.
Yes, it is a stretch, if you haven't gone over 15 miles at once... here's my question: how often are you doing that? If you're doing that every day, you might be okay fitness wise.
What is your route going to be?
If you're riding with traffic, then what kind of attention will you be able to give your riding when you start getting fatigued? When I did my "long commute" - a 50 mile ride to a bike meeting last weekend, and back the next day - I did a lot of work finding a route (the direct route would have been 40 miles... but I'd be more likely to be squished by a car - and basically, it wouldn't have been fun). (And I've been riding 100+ miles per week.) I get stupid when I get tired, so I don't set myself up to have to do a "thinking ride" then.