I think the running/jogging goal would be a great one!
For me, I don't walk. I use the elliptical so it's very different (see Shin Splint post). So my goal of doing the 5k is an easily achieveable goal. I have friends who are trying to get me to do a 10k in 3 weeks. I don't think that will happen. But if you want to do a 1/2 marathon, then go for it.
Our leader asked who were big walkers and people brought up that they walk 3-5 miles a day. It was more inspiring to see who was walking that much.