I wouldn't worry about it, it's a puppy still. Let it be a puppy. At 8 months my APBT/Lab was still your average scrawny looking pup, she only started to 'fill out' at 9-10 months and is still doing so now. I've only just started taking her on one hour walks though, I never had her out longer then half an hour at a go. I'd rather have a healthy dog then a 'built' dog with joint problems. For reference, my APBT/Lab at 11 and a half months.