Note also the fact that 3 miles is almost the same as 5 kilometers. One advantage of a class situation is the group support you get from others of equal ability. This certainly is true with the marathon class I teach in Chicago, but it's also true at every level, from novice to intermediate to advanced. One reason why the Kenyans have been able to dominate the world distance running ranks recently is that they train together and push each other every day in practice. Top runners gather in cities like Eugene, Boulder, Albuquerque and Jacksonville for mutual support.
Group dynamics can be very important in achieving success.
If you have the opportunity to join a class, or hire a coach, or train with other runners, do so.