Chicago to Detroit which is right on the border is about 283 miles, 4 hr 16 drive and around a 4 day walk. Just different terrain i guess but definitely reachable on foot if they can stay hidden.
Most people average about 3mph walking. As J&J are both fairly fit, it seems and aren't carrying any kit, they might do 4mph. However, they might be covering uneven terrain off road (or possibly even unmaintained rutted potholey roads) and will need to hide if they see anyone till thy know if they're friend or foe. Plus they will need to get water, if not food, and rest. If they get blistered feet through not being used to walking such long distances, that will slow them down, as will any ankle sprains or torn muscles.
Even if they walked at 4mph for 16h a day, that's still only 64miles a day. I think if they were doing 40mph on foot, that would still be impressive.
In summary - it's definitely more than 4 days walk. More like a week.