How long is a typical walk-a-thon?

Length depends on the event. If you're doing a walk-a-thon for a specific cause, the length can vary. For instance, the American Cancer Society's Relay for Life events can go up to 24 hours. In that case, team members take turns walking a track throughout the entire evening, with the goal of having at least one team member walking at all times. The Leukemia and Lymphoma's Society's Light the Night event, in my experience, is a 15-20 minute walk. They're both great fun for great causes!

I think usually 7.5 miles.

Most walk a thons have shorter distances, but even if you join a long distance one, you can still participate by only walking or running as far as you can. The important thing is that you made the effort, you tried and you did your best, then you will want to do better the next time. Try it, you will see how these activities inspire. I have seen people in wheelchairs join long distance runs or walks, they go as far as they can and quit. But they are happy because they made the effort and went further then they expected, so even if your walking or running on one leg, or with crutches, you can participate, and you can win too in many ways you never expected.