I had a towbar break during pushback, in a B707 years ago, with three of the four engines started. The first thing I knew about it was the tug rapidly moving away from the aircraft because we were rapidly chasing it, even with only three engines at idle. The brakes came in handy then.
If the engines weren't running and the towbar broke then I doubt whether the aircraft would roll much at all and if it did there should be enough pressure in the accumulator to stop it. There should be enough hydraulic pressure with the engine cranking before lightoff to stop the aircraft.
Regards,
BH.