Someone else might have a more scientific answer, but this will get you going -
Map out a loop that should take you about an hour or a little more - so I'm guessing maybe 15-20kms, depending on hills and your guns. Try and avoid town centers and lights. Don't make it too grueling hill wise, but try not to make it flat. Use this fuckawesome site to do it
http://ridewithgps.com/
Go around it anti clock-wise (faster at junctions) and most importantly
TIME IT so that you get an idea of progress.
Make sure you get a cycle computer (either cheap wheel driven one or expensive GPS) so that you can keep track of your progress during the ride. You'll soon get a feel for what speeds you need to be doing at which points in the ride, and whether you're slacking or just imagining it.
Start doing this circuit about 3 times a week each time trying to best your time. Obviously you can break it up/interchange with other routes, but use this ride as your yardstick.
What you'll now have is a good ride that you can crack off in an hour, probably burn around 1300 calories in the process (half a day's intake), and know whether you're getting better or not. Within a few weeks you can start throwing in other longer routes, still keep this one as your stock-standard measuring stick.
And remember
RULE 10:
It never gets easier, you just go faster. To put it another way, per Greg Henderson: “Training is like fighting with a gorilla. You don’t stop when you’re tired. You stop when the gorilla is tired.”