Invite them to fill some visible role during the hike. Ask to be mentioned on the council calendar, and brought up at Service Unit meetings and Roundtables. A couple will have conflicts. Approach the local ministerial association to explain what you would like to do, and enlist their support. They may need to put this on their church calendars at least 6 months in advance.
Most churches are handicap accessible.
If at all possible, insure that each of these churches can handle all participants. Consider keeping the length of the hike between 4-6 miles. You want younger Scouts to participate, and a long hike will be too much for them to complete. When you can, avoid crossing busy highways. With our growing crowds, we will need to recruit the local police to provide crossing guards next year.