Work situation

Hello Brooke. A distant co-worker that I have only seen 1 time in a year asked me to cover his 4 work day so he can have a vacation with his family. His office is a 6 hour round trip drive for me. I was told by my boss to cover him over 6 months ago for a few days, I did that br hated my life so after I was done I said sorry but I can’t do that anymore anymore. I am already covering 4 offices and a 6 hour drive plus 10 hrs work day is killing me, I will end up in a car accident from inability to focus on the road. They started buying me cheap hotels the night before the night but still working 10 hours plus a 3 hour commute home and wake up early the next day for work are too exhausting for me.
In the last 6 months, this colleague had asked me twice to cover him and I politely declined saying that it is very hard for me to travel that distance and I encouraged him to reach other to other colleagues who are much closer to his office. Now he is asking me again! I feel bad saying the same thing again. I don’t understand why is he asking me while there are 5 other colleagues who live way closer to his office and have been with the company for way longer. I don’t want to go to his office. I want to say no without lying to him but I also want to have a colleague who can do me a favor in the future if I need it. Part of me feels that I need to do this favor to him so he own a favor to me but part of me thinks I am going to hate my life if I say yes and there is no guarantee in life he returns a favor. Please help.

Thank you!