There are two parts to your stopping distance. Your braking distance and your reaction distance, which is sometimes also called “thinking time”.
What does this mean?
Your reaction distance, is how far you will travel in the time it takes to notice a hazard and apply your brakes. Your braking distance is the how far you will travel from the point when your brakes are fully applied and you come to a complete stop.
Both your thinking and braking distances will be longer if you travel faster, so it's important that you travel at a speed than means you have enough distance to stop safely.
There are lots of other factors to consider too, like how dry the towpath surface is, what condition your bike is in, or even if you've had enough sleep – which will affect your reaction time.
But the real question is, do you actually know how fast you're going?
Probably not, you're on a bike, having a great day out. So maybe just play it safe, ease off the pedal power and enjoy the ride.