Good Place in the Boston Area for a Walkathon?

[pedxing]

I've been working with the Phoenix Society (an organization very close to myheart) to organization a fund raising Walkathon for next October. The one site we applied for is already booked for the weekend we would use (A River loop off Storrow Drive near the TV Station and the State Police office).

Does anyone have any ideas of a place hold a 2 - 6 mile walkathon? Even better if you know who to talk to about it.