Help getting Pitbull to run with me

Hello, We just fostered a pitbull (mixed I think) to save from a shelter. He seems to be pretty well trained and loves to walk, has a lot of energy and seems to stay with me when I run.

First run we went about a mile before he started to just stop. Then he ran well for about 2 miles, not really stopping. The third time I ran with him, he barely went half a mile before continually stopping.

I've had other dogs that are runners that do the same and just keep stopping after showing a lot of energy at first.

I know these dogs should be able to run further and I'd assume would enjoy it and certainly need the exercise after staying inside a lot of the day.

I'm not sure why he seems to not like running and continually stopping. I don't want to run with him as this really affects my run; but it would be great to give him that exercise he needs and tire him out.

He is pretty young I think, seems to have been trained as he runs alongside and rarely strays, and doesn't have a lot of other exercise throughout the day.

We have had him for 2 weeks so we don't know him very well yet but he seems to be happy and well behaved inside and outside so far. Still not sure how he reacts to other dogs/people yet though.

Maybe it's me since we had a husky and a German shepherd who were great runners, but after running 4 miles, they seemed to not want to run that far anymore.

I run about a 7-8 min/mile pace.

Any thoughts/advice?