Puppy not wanting to run

My 18 week old neoplitan mastiff cross pup just dosent wat to run, he will walk fast but thats about it ! is it just the fact that the neo is a very lazy dog or is running somthing he needs to understand, off the lead he will just walk by my side, nothing gets him over exited ! lol

At that age a giant should not, and does not need to, run or to engage in any vigorous exercise whatsoever.

His immature joints and bones are very vulnerable to permanent, painful, damage. Until he is around 2-3 years old formal walks should be limited. Do not allow him to jump up and down on furniture, or to jump in or out of the car, do not allow him to run or struggle up and down stairs, do not allow him to engage in rough-housing with other dogs and people.

Be grateful that he does not want to over-exert hiimself

ETA - if you think that there is any physical problem then, of course, you should seek veterinary advice.

Also could it have something to do with the Heat????

Mine can just about manage a walk

yeh i thought about the heat maybe being a factor,

Thanks for the reply, he isnt allowed upstairs or on the furniture so that isnt an issue, he is given a single walke every day not too far more to enphasise training, walking to heel etc rather than his exersise