Corinth was a city in Greece and the readers would be very familiar with the Olympic and Isthmian Games that were held there.
The Isthmian games were actually held in Corinth and the people in Corinth would have probably attended them.
And running a race is an analogy that everyone would relate to.
Any athlete has to dedicate his / her life to just winning the Olympics. Every waking hour was spent in training and preparation.
It takes dedication, self-control and sacrifice.
These are qualities that Paul says believers need to have if we want to receive this crown in heaven. The athlete denies himself things he wants in order to do the things he needs to do.
If we want this reward, we also need to say no to things we might want in order to do what we need to do.
Athletes and competitors keep junk food away so they can eat only that which makes them healthy and strong. We need to keep the spiritual junk out of our lives so we can stay biblically strong and healthy.