This bike will have no problem riding at 60mph for extended periods of time -- granted you have a 400 or 426 motor with upgraded crank pin. What this motor needs is a bit more oil (add an-inline radiator) to avoid more frequent changes, and extend oil life.
With higher gearing the motor will not spin as much, but it will also not be fun off-road. My gearing is at 15/47. Works decent for street hooliganism and off-road shenanigans.
In 5th gear, at 5950 out of 8500 RPM - you are looking at 60mph .
The motor should last you as long as same vintage TW200, or XT225.
Learn to ride. Ride to learn.