Oh, and my suggestion for the site would be to combine something like this with a powerful training log. People aren't going to want to log their mileage twice, so combining the mapping aspect with a useful log is important, IMO.
I use Runner's World log, but really I just picked it because it was the first place I looked for one. It does its job of tracking when I ran and how far, but a bit more statistical analysis would be cool. They obviously have places to enter date, distance and pace, but there are also boxes for weather type, temperature, time of day, amount of effort required, etc. But you have to do your own cross-referencing to get any worthwhile information. I'd like to click a button and see what my average pace is when it's 50 degrees and compare it to my average pace when it's 80, for example. There are sites that do that kind of thing, but I'm too lazy to transfer all of my data.
Also, an easy creation of logs or progress bars to embed or link to (like for a CP signature or a Facebook update) would be awesome. I'm starting to get a little detailed, I suppose.