Not an answer but maybe a step towards a solution?
http://wiki.openstreetmap.org/wiki/Isochrone

I found this, which is a rough implementation of what I want: http://www.freemaptools.com/how-far-can-i-travel.htm

Nice find.

But that uses Google data for route calculation? That's not going to go well for walking, is it?
Mind you, I don't know how good OpenStreetMap data is near you...

Walkscore is pretty good at finding things: http://www.walkscore.com

It's intended to give you a good idea of how walkable a place is (ideally before you move there).

I'd seen Walkscore before. It gave my home a 92 for walking and an 86 for public transportation. Not as good as the 98 walk and 100 riding scores it gives my childhood home in NYC though!

I think you could do this with the old CD based MS Streets and Trips. Well, at least you could do it for driving times - I don't remember if it were configurable by speed or if it just assumed you were taking a car.

Something like cartoo I guess - http://cartoo.dyndns.org/ - though their walking speed seems more like a stroll based on a test in my hood.

Not exactly what you want, but here's how to construct one with Google Maps.

1. Enter your home address.
2. Left click the marker, on the popup, select "Save to map", and save to a new map.
3. Click "Edit" button on left to set title and privacy settings (defaults to public)
4. Right click your house marker, select "Directions from here".
5. Right click somewhere nearby, select "Directions to here"
6. Click on the "Walking" icon on the left.
7. Drag your second marker around - the tooltip shows the walking time to that point.
8. When you have a point you want to save, drop the second marker, left click on it, then select "save to map" from the popup.
9. Repeat 5-8 as necessary.
10. Draw lines etc.

This took about 5 minutes: http://g.co/maps/xq7n6 (not actually my house, but close enough)

@Chet - cartoo is pretty cool. It's neat to see the overlay of walking, then biking, then driving for a given transit time. It doesn't really understand some of the roads (it's got biking distances that aren't really possible due to the roads involved), but it's darn cool.

Seems like an interesting challenge, and not all that complicated with OpenStreetMap data (which actually has many footpaths, which google maps doesn't seem to).. Here's the result of some thrown together code that we're tempted to add a UI to. (The points are the max distances from start point, the tracks are just to highlight them).

Current parameters are: Lat,Lon of starting point, and distance (in miles).

http://maps.google.co.uk/?q=http://desert-island.me.uk/~castaway/kml/sn34xp.kml

This service is offered here for $10. I may try it, will let you know how it goes https://www.fiverr.com/bernie_map/create-a-10-min-drive-time-radius-around-a-location