Fresh Pond is an amazing resource for Cambridge residents. Non-Cambridge residents are of course allowed too, but parking is reserved for residents only. You can also take the T to Alewife and walk from there. The trails are generally all paved, very well maintained (with free Mutt Mitt's!). It is mostly fenced in on both sides so you don't have to worry about your pup running away. The Visitor's Center also provides bathrooms and a water fountain for your pup.
From the main parking lot, you can take the 2.5 mile loop starting either direction. Going counterclockwise, you might want to keep your dog leashed for the first few hundred feet. The paved path follows a busy road and isn't fenced in. You will see many posted signs with the "dog rules" and free Mutt Mitts. They are generally pretty strict with the rules. Along this loop, you will see plenty of runners, happy off-leash dogs, and their owners.
About 0.5 miles in, there is a water fountain for you and your pup if you need to quench your thirst. Then, you will pass the Fresh Pond Golf Course. Shortly after this is Little Pond. This is a designated area for dogs to swim. My pup and I love spending time here. Highly recommended!!