In a world without cars the most likely method of traveling across land will be by foot (or animal, but this still requires some physical acumen).
The fabled "bug out" in a SHTF scenario will likely include some cross country travel on foot after your gasoline storage runs out.
The revered "bug in" in a SHTF scenario will still require you to cultivate the land, bend your back and work animals. All of which require you to be in shape.
Fighting for your life against any aggressor is not like hollywood makes it seem - it is not all marksmanship and sneaking up on people. You need to be stronger than the person who desires to do you and your family harm. You need to be faster than that person. You need to have more stamina to outlast an adversary either on the run or in a CQB situation.
Don't wait - this is not something that you can just start doing once something goes down. It is like taking vitamin C - it does you far less good to start taking it once you are sick, you need to be using it before hand. We all need to be exercsing now. If not for a SHTF scenario, so we can live better, happier more productive lives. All medical personnel agree that exercise, whether aerobic (cardio) or anerobic (weights) benefits our body and keeps diseases at bay. I don't believe in anti-depressants (for myself) -what I do believe in are 4 mile runs (5 if am feeling extra shitty). 45 minutes later I am right as rain - no medication needed. Endorphins are released when you exercise and are truly a force to be recond with.
Lifting weights is nice, but not always possible - do tasks and projects around the house at a level that will challenge you and eventually increase what you think you can do.
I recognize that folks have orthopedic problems (I am one of them) but we just have work around them and find activities/ exercise that we can engage in. This part of preparation is NON NEGOTIABLE.
The best thing about this particlar prep is that is costs you nothing to get outside and run or walk. No memberships needed.