Think we would be capable of 20-25 miles the first couple of days leaving DC. It's pretty level for 90 miles west. We routinely walk 10 miles on the C&O Canal, in 3 hours, carrying daypacks.

3 miles an hour on flat ground.

1 mile an hour on a steep climb (such as a mountain pass).

That's with a backpack and a gallon of water.

Have done day hikes up to 13-miles gaining and losing 2500-feet of elevation. Carrying a gallon of water-Gatorade (it's humid here) but not as much gear as I would in an evacuation scenario (for one thing I'd have to carry dog food in an evacuation scenario).

There are many variables, of course: motivation (radiation or bioterror behind me would provide adrenaline), pack weight, weather, terrain.

I have three mountain bikes and two bike trailers. Those would certainly be preferable to walking.