Oh, man! Your post takes me back to high school! Back then, I read somewhere that 10 minutes of jumping rope in a straight 1-2 rhythm (not the 1-pause-2 rhythm) was equal to 30 minutes of running outside. Don't know how they figured that back then, but I bought it and did it and it really helped my running.
I've recently taken up jump rope myself to teach my 5-year old how to do it. It's ridiculously hard. I'm not sure why it's so much harder to do than running, but I had a heck of a time doing it for more than a couple minutes!! I'm slowly getting better and love it, but it's still harder than running!