Actually very few people can even make a living running. Big races like the Boston Marathon and other high profile races throughout the world have sizable purses. The 2010 Boston winners (M/F) got $150,000 each.
Elite runners are often sponsored by companies like Nike, Puma, Adidas, etc. and receive money for a contracted period of time, and must meet certain standards such as times; entering certain races, do advertising, etc. They also can earn substantial bonuses by winning certain races or breaking World or American records. American runners will run mostly here in the U.S. and Europe. I cannot speak to how things in other countries.
Lesser runners can get a sponsorship that only includes equipment and race entrance fees, and sometimes travel to races.
There are many minor road races where a runner can win $5,000 or more.
In Joe's case he has run the 5K only about 5 or 6 times with a best of 13:57. I think he can get down to around 13:30 very soon. That would earn him $0. If he could consistently run around 13:15, he could make $30K - $50K per year. Go below 13:00 and now he would be looking at $250K - $300K. Of course the runner's career does not last long. A few distance runners make 7 figure incomes. Some sprinters make big money.
For Joe, he wants to see what he can do during the next couple of years, without the pressure of going to school. He will try to find a job that will allow him to work around his training.