It's a free website where you can plan a running (or biking) route and it will calculate distances for you. You add one way point at a time and can undo them easily if you want to change your route, say, to add or decrease distance. You can view map, terrain, or topo versions of your route. The default mode is to follow roads, but you can turn that off and plot straight lines if you are going cross-country.
From the gmaps website:
This is a Web app designed to let you track the precise distance of your workouts, and track and log your activity. It started life as a little hack that uses Google's superb mapping application to help record distances traveled during a running or walking workout, and has grown since then.
As a runner training for a marathon for the first time, I found myself wishing I had an easy way to know the exact distance a certain course is, without having to drag a GPS or pedometer around on my runs. Looking at Google Maps, and knowing there was a vibrant community of geeks hacking it, I knew there had to be a way. So here it is. I placed the a link to my Web app on a couple of running message boards, and the rest was history.
Following Eric Raymond's first law, I proceeded to add features designed to scratch all the personal itches I felt over the years. Features have included: an Openstreetmap overlay, the ability to follow roads as you create routes, create an elevation graph, and log and chart your workouts. The site continues to be under active (if not always rapid!) development.
Give it a try, you will not be disappointed: