Sounds like you had a lot of fun.
I know what you mean about friends rather watching. I found a solution for this. When I started twirling the sling around in a figure-8 style throw, they wanted to know how to do it. Before they would just twirl and release (not successful at all for first-timers).
I have one friend who can't seem to figure out the figure-8. He has a strange prefered "throw". He twirls the sling around vertically, underhand method, (probably about 10-15 times) and then releases. I guess this would be okay... but he launches backwards about as often as he lauches forwards (no worries, just a tennis ball). It's actually quite hillarious. He'll stare up and forward and sort of gaze off like he'd slung the ball 500 feet in the air, trying to find it, while everyone watching [the sling] sees the ball fly almost directly up and backwards. He finally asks where it went and thinks it's funny that it went backwards. I hope he never tries using rocks... [anytime soon at least]
Today in my physics class we weren't doing anything (we've been studying centripetal force) so I spent 40 minutes launching pingpong balls at a poster of yoda on the wall. The poster was about 1 meter by 1/2 meter. I stood about 5 meters away. I hit Yoda 6 times in 40 minutes. I'm sure he used the force!
Just kidding. But it did look like it. I used a figure-8 method and because the pingpong ball's mass is practically nothing and it has a large surface area, it's greatly influenced by wind resistance, so spin plays a big role. My shots would curve out of the way about 1 meter away from him, because the spin put on the ball from the overhand-ish throw (from my right side) causes it to curve left. Anyway, it was a lot of fun.
Wow, you slung for 5 hours! Your arm must be killing you. Were you aiming at any particular target (paper target, tree, pet [jk]) or just slinging for distance?