Rural vote as a percentage of Romney's total:12 million/60 million=0.2=20% (approx.)
Rural population as a percentage of American population: 60 million/313 million= 0.191=19.1%
Not much of a meaningful difference, really. The bigger story here is the fact that many rural Democrats stayed home in 2012, but rural Republicans didn't.
In sum, if you look at the percentage of Romney's vote that was from rural areas, it's actually fairly proportionate to rural America's percentage of the total population.
Of course, the Electoral College kinda screws this up, but in terms of individual voters, Republicans are really no more likely to come from a rural area than anybody in the general population. However, it's also true that rural areas give considerably more of their votes (percentage-wise) to Republicans than cities or suburbs.