If the mind's not strong, the body acts weak, even if it's not. If the mind says it's too cold or too rainy or too windy to run, the body will be more than happy to agree. If the mind says it would be better to rest or recover or cut practice, the body will be glad to oblige.
Running legend Steve Prefontaine suffered a deep and excruciatingly painful deep cut on his ankle the night before the track and field Olympic trials; he competed, won his race with a shoe full of blood, and said afterwards that he hadn't felt the pain of his slashed left heel at all in the race; he had simply told himself that he had never felt stronger in his life.
The narrator is also telling us the same thing in this passage. Our performances, accomplishments, capacities for achievement, are all a case of mind over matter. The body will do whatever the mind tells it to do, so if the mind creates a perception of something that is not do-able, our bodies will recreate that result. The key to achieving physical greatness, or even achieving the physically impossible, is to tell oneself that we can. A positive mind can overcome an awful lot of physical limitations.
It's amazing how two thin pieces of clothing can hold such deep memories. Laughter, pain, victory, defeat, friendship, fatigue, elation - they're all there, but only to the person who's worn the uniform.
Jessica is describing the feeling that she gets from simply looking at her running uniform. To the observer, it is a tank top and a pair of shorts, a uniform that everyone on the team wears. To Jessica, it is far more than that. It is the carrier of all of her memories about her running career. It embodies her love of the sport. It reminds her about championships that she won, races that she lost but learned something from, and friendships with the other members of the team. It's a reminder of that special camaraderie that only occurs with people who are working for a common goal together. Her uniform is a symbol of a life that she very much loved, and that she is now forced to deal with losing.
