Environment and Lifestyle
Environment and lifestyle also play a role and should be consider when you set training goals.
For instance, if you work 80 hours a week and have a family to care for, you may choose a training plan that requires less commitment. This doesn't necessarily mean you can't train for a longer distance, but it may require you to adjust your expectations of how much fitness you'll be able to gain on limited training.
"If someone comes to me and says he wants to run a marathon, but he can only run three days per week, then we're going to talk about why that is less than ideal and what his realistic expectations for the race should be," Leivers says. "I'm not going to tell him he can't do it--we're going to find a way--but with the understanding that maybe he won't run it as fast as he could with more training."
The good news is that physiological, psychological and environmental circumstances can all be changed to varying degrees.
"If you train for longer or shorter distances, you'll become better at those distances," Smith says.
Training is all about coaching your body and brain to get better at a certain skill, whether it be racing a 5K or finishing a marathon. You optimize your individual abilities when you do the proper training for a specific distance. While that doesn't mean just anyone has the potential to be an Olympic sprinter or marathon runner, we can all excel at distances we are supposedly less suited for with the right training.
In helping his athletes decide what distances to focus on, Leivers says that he always emphasizes that "stress is stress." Whether it's from training, working, sickness or family commitments, your body perceives all stress in the same way. In other words, we only have so much to give.
"So while all those things may not dictate what race distance you train for, it absolutely should dictate how you train for that race distance," Leivers says.