I think you can show up the day of.
I did the power walk twice. Once with the gal (she walked slower than the guy; that or I was just more up & peppy when I walked with the gal). With the guy, I kept on being left behind in my power walks and the group had to wait for me. Once a truck was backing up so I waited until the truck finished backing up (no way was I going to walk behind a backing up truck).
I signed up for the power walk with the gal via the phone (just called the phone # that Disney said to call). I impulsively decided to sign up for a power walk the second time so just showed up.
I don't think they take attendance or anything.
Also, anyone under 12 (?) cannot do the power walk (the age requirement is the same to use the fitness facilities). BTW, anyone from 13 - 17 that signed the waiver - that's not valid (kids cannot waive anything; their parents would have to do that, but that's a different story).