With a decent controller, one can control the speed with the pedal assist level, but it sounds like your kit doesn't do so, as it doesn't have multiple levels, and the default level must be too strong. Does it go to max speed as soon as the pedals spin? I had a controller that worked like that, Had to scrap it.

You just have to get used to holding the twist throttle like a motorcyclist. If you're electrically inclined, you can wire in circuity to limit the voltage on the throttle as needed. The throttle output changes from 1V at rest to about 4 Volt at max.