This USGS map from the early 1900s shows the path of the aqueduct; the hikers walked along the route I've marked in green, starting at bottom.
The web site has lots of photos. Here are a few of them.
1. This shows a ventilator, one of more than forty that dot the route, each about a mile apart. Ventilators kept the air below from fouling and prevented air pressure from building up in the tunnel.
2. This shows how earth was mounded up over the tunnel.
3. Here's the dam. It's not the original one however. The first aqueduct was replaced with a second one in 1890; that one's still in use.
4. This painting, from the town's historical socity shows what Ossining looked like at the time the aqueduct was constructed. It's "Village Of Sing-Sing," 1830, by Hugh Reinagle.
5. Here's another painting of the village, made ca. 1839.
6. This map shows the village half a century later, considerably enlarged but still calling itself Sing Sing (it wouldn't become Ossining until 1901).
Croton Aqueduct article in wikipedia
Ossining article in wikipedia
Ossining Village History
Ossining Historical Society