Well since you adopted him who knows whats actually in his genetic make up, he may not have any American Pit Bull Terrier in him whats so ever.. It is very common for shelters of all sorts to mislabel dogs as they are not dog breed experts, they best guess the breed and move on. Now i am not saying your dog has no APBT
in him however the likely hood is low as APBT
's as a whole are not only very careful as to who their pups go to but also rarely escape and mate with an unknown dog to end up at the shelter.. It doesn't NOT happen but its low compared to other dogs.. Usually American Bully/mixes, American Staffordshire Terrier/mixes and Staffordshire Bull Terrier/mixes are the more common of breeds to end up at the shelter in terms of dog breeds that fall under the "Pit Bull" radar.. It can also be said that Boxer mixes can mimic very closely to the fore-mentioned breeds and can easily confuse someone who hasn't been dealing with this particular breed for years.
Now with that said its all about genetics in terms of how conditioned/toned/etc your pup can be. While most dogs can get in relatively great shape no one dog is created equal. Some dogs can be conditioned and have "lots of muscle" with very little effort involved, others can be worked every day for a few hours and never really have a conditioned body. Fit, yes but not conditioned in the sense of the word.
Flirt pole, spring pole, hand walking, swimming, tread mill, jogging/running, etc can all create a well conditioned, properly fit dog. The key is to know your dogs limits and not over stress the body, often with these dogs you will have to force them to quit any given exercise as they tend to forget the meaning of "slow down" and once they are going its full steam ahead. You also want to play around with various exercises until you start to notice results, not one dog is alike in that what works for one may not work for another.. Again, a big part of this is genetics but also the drive and over all will of the dog will vary. Some love flirt/spring poles, others look at it and move on. Some love swimming, others would rather just drink the water.
Pace yourself and your pup, never start out hard and ease the transition from little exercise to a good load. Especially in the beginning you want to make sure you allow some down days where little exercise is given, you never want to over work your dog as this can result in injury..