Any combat soldier will tell you that when the fighting breaks out, he automatically falls back on his training to get him through. It's the training that becomes instinctive through countless hours of drills that keeps you alive when the going gets rough. If you have no training, you have nothing to fall back on.
The same goes for a Plan. If you have a survival plan that you have rehearsed and practiced over and over again, chances are your brain will revert to that plan when you need it to save your bacon. If you have no plan, you have nothing to fall back on.
There should be a Primary Plan and at least one Contingency Plan. The primary plan is the one to use when everything goes just right. Contingency plans are the fallback strategies to use when the primary plan fails due to unforeseen circumstances (which are the very essence of a disaster).
In a military sense, when the enemy shows up on your landing zone where friendly choppers are scheduled to pick you up, the primary plan is scrapped and the first contingency clan put into operation. You make radio contact with the chopper and tell them to head for the first alternate landing zone. Notice I mention "first" when I talk about contingency plans such as alternate landing zones. You always need more than one contingency plan, because your first option might also run amok. Then you head for your second option.
In survival planning, you need to outline what you will do, where you will go, and how you will get there (and all that kind of stuff) in a primary plan. Hopefully, when you put that plan into action, it will go well. But as often as not, you need to change something(s) on the run, and that's where contingency planning comes into play. For example, if your primary destination is still viable but your means of getting there is not, then pick a new method or transportation from your contingency plan. The same concept applies to all aspects of the survival plan.
It isn't a plan until you write it down. Until it's on paper, it's only a concept. Once you put it down in writing it becomes a tangible plan that you can run through in your mind and use the eraser to adjust whatever need to be changed. Keep doing that until you think you've got it right. Then build first, second, and third contingency plans that will give you options in case Plan A doesn't work out.
Practice all the plans mentally, and then do simulated run-throughs to make sure you can get from Point A to Point B in the amount of time you have planned. If you never go out and actually follow through on the plan, you won't know if there is some new obstacle in the way that would prevent that plan from working. Maybe a road that you were thinking would be a viable route has been torn out, etc. From time to time, trace your routes in real time so you can be aware of changes.
Share your plan with all family members, so everybody will know where the primary and alternate rendezvous points are when a crisis hits.
With a plan, you have a chance. Without a plan, you're probably not a survivor.