How to draw a travel time boundary on Google Maps

If you have a specific destination in mind, then Google Maps is perfect for estimating how long it will take to get there, by various modes of travel. But if you just want to know how far you could travel from a given address in an hour, then it isn't much help.

Perhaps you're visiting somewhere new and just want to see what you could walk to within an hour of your hotel. Or you want to make a day trip somewhere and want to see where you could drive in a couple of hours.