
The Mystic Link Trail runs along a narrow bit of land along both lakes, and this is the Mystic Lakes State Park. The other side of both lakes is mostly private land so you want walk all the way around the lakes, but there is lots of good walking on the eastern side.
More Resources:

All Trails for the Mystic Lakes
Recommended trails and map

Commonweatlh Page
All the rules and regs for the lakes