summer to run. He figured if he worked at it - and Lord, had he worked-he could be the fastest runner in the fifth grade when school opened up. He had to be the fastest-not one of the fastest or next to the fastest, but the fastest. The very best.

Jess first waits until he hears his dad's truck start up, so he knows that his dad will be out of the way, and while his mom and sisters are still in bed, Jess dresses quickly and sneaks outside. Then he runs for all he's worth, pushing himself hard to get faster and stronger.

Running is a big deal among the other kids at school; it's a way that the kids compete against each other and determine among themselves who's the strongest and who's the best. Because Jess has never been a fantastic student or an excellent athlete in other sports, either, and because he's most interested in sitting quietly and drawing alone, he feels a need to excel in the races at school so that he can make a name for himself and improve his reputation. During the previous school year, Jess had won a race against some other kids and had a taste of the glory and admiration from others that this victory provided. He wants more of it!

The answer to this question can be found in the first chapter of the book.  Jess is one of the story's main characters, and readers are introduced to him when he wakes up in the early morning.  Jess wakes up, hears his father leave, and decides that it is safe to get out of bed.  One of his sisters asks him if he is going to go running.  Jess responds with a "maybe," but readers get to listen in on his thoughts in the next paragraph.  Jess explains that he has been getting up to run every morning during the summer in order to train.  He wants to be the fastest runner in all of the fifth grade.  He figures that his natural talent, coupled with dedicated training, will see him to victory as the fastest fifth grader in school. 

Of course he was going to run. He had gotten up early every day all summer to run. He figured if he worked at it—and Lord, had he worked—he could be the fastest runner in the fifth grade when school opened up. He had to be the fastest-not one of the fastest or next to the fastest, but the fastest. The very best.

Unfortunately, Jess's plans are wrecked by a new student that happens to be faster than anybody else.  That student also happens to be a girl.  

She [Leslie] beat him. She came in first and turned her large shining eyes on a bunch of dumb sweating-mad faces.

Jess has gotten up early every single morning all summer long. This is definitely not normal behavior for a soon-to-be-fifth grade. Jess is getting up early to train to be the fastest runner in the fifth grade. He longs to impress his friends, families, and teachers and wants to feel accepted and fit into a "mold" of some sort. So, he hopes that running will allow him to fit into the mold. He is also aware, on some level, that everyone (even people who aren't athletic) will be impressed by his ability to run quickly.

Jess has gotten out of bed early every day all summer so that he could go running. Jess is a pretty good runner, and he figures that if he practices, he has a chance to be "the fastest runner in the fifth grade when school open(s) up." Jess is determined "to be the fastest - not one of the fastest or next to the fastest, but the fastest. The very best."

At Lark Creek Elementary, where Jess attends school, there is not a lot of athletic equipment for the students to use. The few balls that are available are monopolized by the older children, the sixth and seventh graders, so in order to amuse themselves, the lower-grade boys had started "this running thing." The boys would all line up on the far side of the lower field, and at a shouted signal they would race "to a line they'd toed across at the other end." Jess, as a fourth grader, had won this race one time last year, and the triumph "had put into his mouth a taste for winning." Jess has had a reputation from the time he was a first grader as a quiet student "who draws all the time; "he had surprised everyone when he won that race, and he enjoyed the attention that resulted. Jess figures that he has a pretty good chance to be "the fastest kid in the third, fourth, and fifth grades" this year, and is determined to practice enough to make that dream a reality (Chapter 1).