Castle Island Loop

This loop hugs the HarborWalk the whole way around, so you get open water on one side pretty much the entire run. It's flat, it's paved, and it's wide enough for two runners and two dogs to pass each other without anyone doing the awkward leash shuffle.

Fort Independence sits right on the far side of the loop — a real 19th-century fort you run straight past, not around. It's the natural turnaround point if you're splitting the loop with someone, and a good spot to let dogs sniff around while you catch your breath.

Castle Island is a South Boston institution, so expect company — other runners, walkers, plenty of dogs. The paved surface makes it easy on paws and fine for strollers too. One thing to know: it gets windy off the water, especially in fall and winter, so bring a layer even on a mild day.