I've been road walking for many years. I consider it as much fun as backpacking. However, I've never taken an overnight trip by road.
This September, a friend and I are planning a 47 mile (round trip) weekend to a nearby town. This is the next step in serious preparation for a much longer trek.
Since I got a pushcart, miles don't count the same as they do backpacking in the mountains. I'd say 3 miles on the road counts about the same as a mile in the mountains. Weight doesn't matter as much either.
I did some testing with a fish scale. It takes about 3 pounds of horizontal force to push a 30 pound load in the push cart.