I totally agree with Teslinhiker's choice of footgear - low cuts are much preferable to high top leather shoes. Their lighter weight will significantly increase the distance you can hike, as well as your comfort. I would recommend after market insoles, like Superfeet or Spenco (my preference), especially on concrete surfaces.

I know your circumstances apparently rule out using a bicycle, but this may not be the case for others. They should consider that while 20 miles is a long day's walk, it is an easy two hour bike ride. I can consistently average ten mph with a fully loaded touring bike, covering seventy to eighty miles a day, carrying enough gear to remain self sufficient for days at a time. If equipping for a GBH situation, the load could be decreased and bike durability enhanced to deal with the complications preset in a post earthquake situation.

I also live in earthquake country, but my part time job is only five miles away. I usually commute on my touring bike, and I always have what is required to walk the distance if necessary.

Congratulations on being proactive about this situation.