I'm thinking 60 miles on battery only is going to require about 30Ah on the battery. And that's probably only at 20mph. You only need a 500watt hub drive to get to 20mph on throttle only.
I have a Magnum Metro and can go 24mph with easy pedaling, and 20mph on throttle only. I haven't tested the limit, but with pedaling to assist the motor I can be at 80% battery after a 20mile ride. However, I can't comment on the accuracy of the battery remaining indicator without a longer ride.
Typical batteries are 10-17Ah, so as you can see it's most likely you would need to carry a spare pack or find a way to "build your own" solution. The 30mph on battery only is not really a target market, so the likelihood of finding something like this off the shelf is tough.