Water pump leak - how fast?

I have a leaky water pump on my 405 TD but need to drive 150 miles, mainly motorway. Never faced this before. Questions:

1. Can the leak get worse very fast?
2. Will the temperature guage - reliable up til now - get unreliable if the water gets very low?
3. Does speed make a difference to rate of leak?

Obviously I want to set off with gallons of water on board and check and fill up at intervals as necessary.

I've already put in K-Stop? K-Leak? but it didn't help much.

Thanks.