I must admit, the first time I read this short story I didn't automatically see the connection between Nick's activity in the forest and his healing from the scars and trauma of being involved in the war. However, if we look more closely at Nick's experiences and his activities, we can see that Nick is a character who is desperately trying to escape the real world with its series of obligations and the need to think about what has happened to him. Note how the text describes Nick's happiness:
His muscles ached and the day was hot, but Nick felt happy. He felt he had left everything behind, the need for thinking, the need to write, other needs. It was all back of him.
Healing for Nick comes from this space in the natural world where he is able to engage in a series of physical tasks that distract him from thinking about what is going on in his head.
Later in the story, we can see that the swamp he finds where he knows there will be bigger fish, where fishing is a "tragic adventure," is equated with returning to the responsibilities and care of life. It is highly significant therefore that Nick does not feel able to go to the swamp at this stage and stays by the river. The story ends with the following sentence:
There were plenty of days coming when he could fish the swamp.
This indicates a future where Nick will be able to go back into the real world with all of its cares and responsibilities, but for now, he needs to stay free from those whilst he heals and processes what he has experienced.