My wife and I built a 160 foot long zip line on our property. It runs down into a gulley, about a 20 foot drop. Instead of connecting the ends to trees, we put in a post at the start, and 2 at the end. We used 12 foot long 6x6 posts, and buried them 4 feet into the ground. We used 5 bags of fast-setting concrete for each post. We also used two 4 foot long 4x4 posts at each end, buried 3 1/2 feet deep, with 3 bags of concrete each, to connect guywires to for added support. We used 2 posts with a crossbeam so that we wouldn't crash into a post at the end of the ride. The cable is 1/4 inch galvanized aircraft cable, and all the eyebolts and turnbuckles are galvanized. We made a brake from a wooden block mounted on the wire and tethered to a tree with a bungee. There is also a wheelbarrel tire on the cable before the turnbuckle to help absorb any leftover forward motion. The ride lasts about 9 seconds; I'm guessing the maximum speed is about 24 MPH. We've got a swing seat and a handlebar, as well as a harness.
Looking up from the end to the start This picture shows the 2 posts, the crossbeam, the turnbuckles used to tighten the cables, the tire at the end of the line, and the brake.
The end of the ride This is the opposite view, showing one of the guywires.
Brake In this picture you can see the brake bungee more clearly.
POV 1 A little more exciting looking from my point of view.
POV 2 I rotated on the way down on this ride, hitting the end facing the start. There's a shot at the end of the trolley on the cable next to the brake.
The cable and hardware came from Zip Line Hardware
The posts came from Lowes and I got a cheap come-along from Walmart and a Haven's Grip from Amazon to tension the cable. I also got bolt cutters from Lowes to cut the cable.