He fights too much water, too little water, bugs, and weeds. He waits patiently to see his crop mature, but when and if a harvest is brought in, the satisfaction of a successful harvest is overshadowed by the prospect of beginning the process all over again. I understand the point Paul made. I have known the thrills of the ministry pictured by a teacher, soldier, and athlete. That requires a willingness to work hard without thrills.
Many today are reluctant to work hard in the ministry.
Even with that kind of commitment, the crop often doesn't not come in as you expected. There's hard work but verse 6 also makes the point that when the crop does come in, the farmer ought to receive the first of the harvest. Few work themselves to the point of exhaustion, but only they will experience the blessings Paul speaks of here. Great reward from God comes only from great labor for God. In the future it will be the privilege of all hardworking servants of Christ to throw their great rewards at the feet of their Savior.