If you’re good with the idea of doing it for charity (and the fundraising that goes along with that!), I highly recommend Team in Training, which benefits the Leukemia & Lymphoma Society! I’ve run 2 half-marathons with them and found the group to be very motivating and the training excellent. For most half-marathons, they have a dedicated Walk Team that trains together with a coach and/or mentor who specialize in walking races.
If you contact them for information, they should be able to tell you if they plan on having a Walk Team for upcoming events. It depends on registration I’m sure.