Ignoring the bit about walking for the moment, the underlying issue is absolutely serious.
In short, modern agriculture works only with an ongoing fossil fuel energy input. That is very different to traditional agriculture relying on the sun only.
Given that not even the optimists expect the abundance of oil and gas to last more than another few decades, the ability to keep 6.6 billion or so people on this planet would seem to be under a very real threat. Traditional agrigulture produces food, but not on the same scale that we're eating today.
This issue ranks well ahead of terrorism, interest rates etc in importance in my opinion. I'm not worried about having to drive less (or not at all). But not being able to continue eating would be a rather big problem...