Overall, this is an interesting story and is pleasant to read. I enjoyed the running theme and felt that I got a small insight into the sport and how it can feel to train with others.
As I enjoyed the insight into running, I would like a little more. You have an opportunity here to use some gentle jargon about the sport. Readers don’t like to be confused, but they like to know the words they are reading are authentic. Therefore, I would suggest you put in some more details that are specific to running. For example, you could comment on the type of trainers the protagonist is wearing, and how they feel on his feet. Maybe he bought some new ones as he knew he would be training with this new group of people. Also, perhaps he might be considering his strategy about his position in the group while they are all running. Small details like this will interest the reader and make them feel they are learning something about the sport.
Be careful with grammar. Try reading the piece out loud to yourself and you may find it easier to see where grammatical mistakes have been made. In particular, your use of semi colons needs work. A semi colon joins two related concepts. However, each group of words must be a complete sentence. In other words, there should be a complete sentence both before the semi colon and after it. If either of them are not, then a semi colon is not an appropriate piece of punctuation to use in that case. Along the same lines, when writing dialogue, the rule is that if a new person is speaking, you must start a new paragraph for his dialogue. So, in the conversation between the coach and the protagonist, start a new paragraph each time one of them speaks to the other.
I liked the character of the coach very much. I felt I could hear his voice very well. I would like some more physical description of him, as well as the protagonist, actually. Furthermore, I enjoyed the conversation between the two of them and would like it to be longer. Perhaps they could discuss something deeper, like why the protagonist is so quick to say he wants to quit running. There must be a reason for this flaw in his character – perhaps he has had a difficult upbringing which causes him to give up on things quickly if he is finding them difficult. Details like this help the reader to get to know the characters on a deeper level and this, in turn, will make them enjoy being with the character even more.
Also, I would like to see the conversation between the coach and the other runners in real time. Rather than just reporting that it is what he did, show the scene to the reader. The dialogue and the reactions from the other runners would give readers even more of an insight into the sport of running and into the coach’s character.
I enjoyed reading this story. It is a good central idea, but I would like to see more of it. More details and more dialogue would give the readers something more to hold on to.