Dear Brendon,

I’m very glad to hear the video is helping. The cramping and tiring is pretty normal at first, but you should check whether your hand is positioned uncomfortably, or is tense in any way. That could be affecting it. Experiment a little to see if it works better with a bit of relaxation, or slightly different hand position. Otherwise, it's just a matter of the muscles that control the fingers getting stronger, which comes naturally with time. I don't know what to suggest as warmup exercises, as the rolls you're doing are what some people do as a warmup. Try doing them a little more slowly at first, without the video running. I'd say just a few minutes of that, and then try playing with the tape. It should get easier in time.

Best of luck with learning the banjo!

Pete Wernick