It's Only Temporary (Paperback) by Eric Shapiro

After Sean graduates from college, he is ready to begin his life. Silly him for assuming he will have one. Word gets out that humanity is doomed: a giant meteorite has been headed toward the Earth for decades, & the government is ill-equipped to stop it. On the final day of human life, Sean must decide who to spend his precious time with. When he opts to hit the road & reunite with a lost love, he encounters a mad society that is rapidly shredding the trappings of civilization.