If you have an Iphone, you can use the free app on mapmyrun to track your route and work out your distance when you get back! I used it for running, but when I got serious I found it was a bit inaccurate.
You can even buy a device called a garmin watch which is even more accurate, but that costs dough. It can help you with pace, lap times, etc.
Good run guide is also very good, but I think they charge a fee for their app.
Cheapest way is the way I now do it. I know my routes, how many miles they are, so I just use my trusty Casio which was £9.99 and work it all out when I get back.