Google maps will do it for you - choose one location, then click on Get Directions and enter the other location. Above the locations are buttons for car, public transport, walking and bicycling.
Google Maps lets you change the route by dragging points on the map, and the total distance is given in the summary at the top of the individual segments of the route.
if you use googe maps you can put in walking and then drag the blue line on the map to the route you want to take.
If you want to walk across fields or anything else that wouldn't normally be considered a normal route you can use google earth (the one you download, not the on-line maps) and work out the distance using the measuring tool.
ChuckFickens - just tried dragging the blue line on the Google Maps site and yes that does work, although only from A to B, ie doesn't work for a circular walk, eg. A to B and back to A again but via C. However, by breaking the route up into segments, it does the trick. Cheers