I haven't see anyone bring up chemical and biological causes for a SHTF, unless you consider the nuclear that was brought up.
When Katrina was happening, I told me wife if we where there, we would be driving out, if we could not drive out we would be walking out. Healthy people should easily get 10-15 miles a day.
Looking at ab28 thread on hiking the AT, there were days he made 30 miles on the trail.
Knowing your area, your starting point and having an idea where you want to end up would be must. Knowing a couple different routes to get there would be a plus.
Since main roads would not be the best choice, I have been considering railroads as they are normally more direct. However I am sure many other people have thought of this as well.
If it is chemical/biological/nuclear/other, step one is being in the initial survivor group. You may have very little control over this part. After that is when your additional planning would come into play.