If a gas station is 1 mile away from my house and the average person walks 5mph. Its a sunny day and no wind. Why does it take my dad 15 years to come back from the gas station?
Well, there're a couple easy answers.
First of all, let's concentrate on the word "average". Average person walks 5mph, but your dad isn't average is he? Oh no, far from it. So any length of time will be possible.
Another solution is "house".
The gas station is a mile from your house, but where are you and your dad at? Another country probably. It's gonna take him some time to go all the way to that gas station and then back to where you are...
By the way, I saw some comments about people calculating how long it will take to walk around the Earth.
Surely you don't expect a human to walk non-stop 24 hours a day? A hard hiking pace is about 20 miles per day, less if you're going uphill. Then you'll have to consider all the food you'll need to eat. Does the dad have money to just buy everything, or will he need to hunt? There are places that don't have any stores around, so he'll have to hunt / gather. Weather conditions may vary. The pace is dramatically reduced for all these considerations.
I think that he thought the gas station a mile away from you was closed, so he went to one that was open, which happened to be on the other side of the earth. OR the gas station around all had crappy gas quality, so he went to the other side of the earth for you. but there also is that possibility that he just doesn't like you.