I walk 3 miles to work and I often walk 3 miles back as well. I carry: a small, light laptop and charger, glasses, at least one book, a purse, makeup bag, phone, and an umbrella (if rainy). It's fairly heavy -too much to be managed in an overarm bag or messenger bag.
I need a reasonably heavy duty backpack with different pockets I can put all this crap into! It needs to be light and able to cope with a fairly itinerant daily schedule too. I would ideally like something that looks reasonably professional, i.e. thatdoesn't look like I'm about to embark on a Duke of Edinburgh-style adventure. 
I currently have a Fjallraven Kanken, but it's just too small and having everything in one pocket is making it annoyingly hard to find things.