Generating the amount of corn-based ethanol won't be a problem, but alcohol derived from food crops is significantly less attractive a solution than cellulosic ethanol, which can be formed using waste products like switchgrass and wood chips. It is indeed possible to create cellulosic ethanol today, but not cost-effectively and not in the large quantities necessary to meet the looming requirements.
There are options available to meet the deadline. Allowing foreign countries like Brazil, where ethanol is distilled from sugar cane, the right to import larger quantities of fuel into the US would do it. Alternatively, if the United States isn't able to reach its ambitious goal, there are provisions built into the standard to adjust it down accordingly. Not much of a mandate, then, is it?