I was recently trying to map out a walking tour that I once did in Rome, and in the process I discovered Mappy.com. It's a European site that allows you, among other things, to map out your own tours by entering the locations you want to see.
You have tons of options, too - you can visit the places in the order you enter them, or have the map "optimize" for you. You can choose your method of travel - car, foot, or bicycle. And you can see it on a map as well as have written directions. It also calculates the amount of time and distance your itinerary reflects!
I did a quick sample one for Rome when I was writing my last post, and did a tour starting at the Colosseum and ending at the Spanish Steps. Stops along the way include the Mouth of Truth, Castel Sant'Angelo, Piazza Navona, The Pantheon, and Trevi Fountain.
This specific trek is 7.7 km long and estimated to take 2 hours... obviously we'd take longer so we could stop and see stuff, detour, take photos, eat gelato, etc. But I really love this site, and I have a feeling I'll be using it whenever I want to customize a walk!