Prior to 1954, experts believed that running a mile in less than four minutes was impossible. They believed that was a bit beyond human capability.
Some believed that even if someone could do it, their heart would explode. However, Roger Bannister broke the four-minute “impossible” barrier and clicked in at 3:59.4 in 1954.
The remarkable thing is what happened immediately after that: just over a month later someone else did it. Soon after that, a ton of people were doing the "impossible" sub-four-minute mile. This is now the standard of all male professional middle distance runner. There hasn't yet been a woman who has run a four-minute mile. As of 2014, the women's world record is held by retired Russian Svetlana Masterkova (time 4:12.56).
This is a great example of the real barrier all along being psychological.
Take a long look at your life. Do you have any four-minute mile beliefs? Do you dare to break them?