I've never been on a bullet train. Will it be very tiring to travel to Xiamen for the first time tomorrow and take the bullet train for more than five hours?

No, the seats on the train, the temperature and so on are quite comfortable. If you don't dislike weight, you can bring a book that interests you. Pick it up when you are bored, or look at the scenery outside the window and listen to music. Five hours passed quickly. I sat on the bullet train for eight hours, and it was the same.