I came across a really cool tool this afternoon (via Penmachine) – Gmaps Pedometer! Basically its a tool that someone named Paul cooked up that lets you draw a route on Google Maps and it will calculate the distance for you.
I started playing with it and found myself getting addicted! So you have been warned, this tool might lower your productivity as you try to figure out the distances of all the little routes you take. For example, the route I take to the office from home is about 14 KM, while the route from the office back home (I go a different way) is about 12 KM. The tool only displays distances in miles, so I converted them using Google’s calculator.
Give it a shot! You might discover that the route you take is longer (or shorter) than you think! I am not entirely certain how accurate the results are, but they seem pretty reasonable to me.
Read: Gmaps Pedometer
MasterMaq's Blog 
Incredible! I’ve been spending the last 30 minutes playing with this, measuring distance to work and my jogging route. As you said, truly addictive. By the way, you can now measure directly in km.
Awesome, thanks for the update!