As this video from Cedar Wright shows (he’s more than dabbled in highlining!), slacklining is a great form of physical and mental training. For any form of outdoor adventure.
I taught myself to slackline when I moved back to Iowa to finish undergrad: I spent the better part of a very snowy February and March flailing in approach shoes (it’s much easier barefoot) before I could consistently mount the line. In the end it’s like riding a bike, once the mental and biomechanical breakthrough moment occurs you’ll never forget how, just get rusty if you go a while without doing it.
A 25 or 30 foot line is the best to learn on. Too tight or too slack are both harder than a rig just in the middle. All you need is 40′ of 1 inch tubular webbing (thinner stuff stretches more, an advanced trick), a few locking biners, two stout trees, and a basic understanding of rigging. Hint: tie knots that are easy to get undone (butterflys).