Running difficulties

Question
I just recently adopted a 2 year old boxer mix from a shelter. Pulling during walks is probably what I have been struggling with the most with him, heeling is something that he is not getting. I am a runner so I decided to start jogging with him. He seems to pull less when we are going at a faster pace, but about 5 mins in he will just suddenly stop and refuse to move. We wait for a min then start up again. We did about 2 miles today at a very mild pace and stopped frequently. He stopped towards the end and just suddenly laid right down. After we got back he just plopped down on the floor and when he tries to get up he can only move a few steps before collapsing on the floor again. He is about  63 lbs, and has a very muscular build, not overweight. I'm afraid he is in pain or there is something wrong with his joints. Please Help. Thank you.

Answer
It may well be joint pain - just like a human athlete, he needs to build up to running any distances, and the repetitive pounding is very hard on the joints.  

You don't mention where you are, or what the weather was like.  If your dog is Boxer-ish in head type, he will have low tolerance for heat and humidity - long walks and runs in this weather can be problematic.  (In extremes, of course, even full-muzzled dogs can have problems with heat and humidity.)

Another possibility is a heart issue, to which Boxers are unfortunately very prone.  I don't supposed you noticed the color of his gums when he stopped, and when he 'collapsed'?

I would definitely say a vet visit is in order, to rule out joint damage or a heart issue, if you want to keep running with him.  As well, here are a few articles about developing an exercise program for your dog:
http://www.caninesports.com/fitness.html
http://www.squidoo.com/runningdog
http://www.paw-rescue.org/PAW/PETTIPS/DogTip_JogDog.php

As far as pulling during walks, there are several good articles on that subject (Loose Leash Walking) here:
http://www.clickersolutions.com/articles/#behaviors