Interesting, I have always walked for measures of time, and not distance.
It is interesting how different people look at things differently.
I have been a walker for quite some time now, in fact don't even own a vehicle,but I do, however use mass transportation.
Yesterday, I walked for 3 hours 10 minutes and had a backpack with 8 lbs of water in it.
At the end of the first hour, there was an immense hill that I climbed for about 30 minutes or so.
As a reccomendation for you,why not start with 2 slow 30 minute segments per day, and if you feel you can do more, either pick up the pace or add more time.