This past Labor Day weekend, I did a 4 day outing that averaged 18-20 mile days over easy to intermediate terrain along some well established trails, firebreaks, old Forest Service access roads etc with elevation gains/loses of around 1000 feet each. Even though I am in very good walking/hiking condition, there is no way I would want to attempt this for 400 miles while carrying my 40-45 lb pack every day even on a good road surface such as concrete, asphalt or an all weather gravel road. As it was,I suffered from a very rare occurrence of blisters on my right foot that could of been an ugly debilitation if this was a forced 400 mile foot march.

Also the OP did not mention of water that needs to be carried. On a couple of days of high 80's temperatures, I was drinking upwards of 2 liters per hour and could of used more if I had it to spare as this time of year, the area I was in, many creeks and streams that just a month ago were overflowing their banks with mountain snow runoff, have all but dried up. I had to keep water consumption in check as these usual water sources were no longer available. Conversely in other areas, especially in some US states, severe drought conditions would make water supply that much harder for one person, let alone multiple people in your group.

Like others have similarly commented, the likelihood of anyone having been forced to do this to get to Grandpa's farm in a bug out situation is not realistic in terms of logistics, physical exertion etc in the terms of the OP's scenario.

People just do not really realize how far 400 miles is. Go to Google Maps and do some mileage calculations. For example, Denver Colorado to Albuquerque, NM is just shy of 400 miles. Columbus Ohio to Nashville Tennessee is about 400 miles. Once you get a sense of scale and area that needs to be traversed, are you really sure you still want to undertake this bug out Grandpa's farm?