Dear Sam,

Hope you're doing great. I've got your recent letter and learned about the problem, which you seem to be currently facing at your work. Of course, I feel a bit frustrated about this issue of yours, but please understand that this kind of problem is not very unique. 

In fact, I also had to deal with this type of problem about a couple of years ago when I was also asked to visit our manufacturing plant to train some junior staff members in a different city almost every other day, even though it was never a part of my job description. But, I kept doing it as long as I could, but then went on to prove to my upper management how the quality of my regular tasks was being affected rather negatively as I was getting a bit too tired of the long travel to finish my regular tasks on time.

Luckily, the upper management understood my problem as I spoke to them very candidly. I suggest, you also talk to your superior about your problem with some facts, and I'm sure that they will listen.

I hope that these advice would help you in your problem.

Gregory