Well, not to be a smart-a$$, but if you're planning for an engine fire, do you have a fire extinguisher?

Your kit looks pretty good already. Ditto a little better shelter. But 20-3 miles is a very do-able 1-2 day trek with a light pack - assuming decent weather. I'd add a 2nd quart of water and maybe some way to purify more, since it's amazing how fast that can go on a hot day.

I'd worry more about where the kit is located - next to you, but is it strapped down? Ie, if you start rolling, will it end up in wedged under a chair somewhere where you can't get it? If you keep it strapped to your truck bed (roll bar?) can you get to it easily?

Personally, I don't think I'd off-road in sandals, but its your call. Maybe make a habit of wearing pants instead of shorts (or maybe the zip-off pants, and keep the legs in your bag) for a little better protection.