Yeah, definitely keep the horse, it's a long term investment.
In the short term you have to prepare for the long term.
I think there would be more 'road warrior'-esque gangs than you'd think, and eventually they would move over to horses too, and eventually back down to swords and bows until communities have re-established themselves enough to start getting things working again.
Winter is also going to be a big factor. I'd say the first few winters post-event will be a fair bit colder than usual because of all the smoke and dust in the atmosphere from the fires burning out of control. Particularly in things like oil refineries and the like.
Which brings me to another question...
What would happen to the nuclear power stations?
