Genetics can be a real pain in the butt sometimes. As much as you're genetically predisposed to a certain physique there are also ways in which you can pick your physique. How we train influences how our body adapts. Plain and simple, the type of exercise you do matters.Take a good look at fitness magazines and athletes that you believe have the ideal body. Are they all slim and trim but with no real muscle mass, are they gargantuan bodybuilders with massive arms and legs that look like tree trunks? Or are they somewhere in between, lean but muscular, well defined but still believable? Whatever you view as the goal for your physique, take a moment and look at how those people train.
For instance, let's look at a defensive back in the NFL. Most of them are between 5'11'' and 6'1'' and weigh in around two hundred pounds give or take a few pounds. They are often lean, well muscled and extremely strong for their size. They are also lightning fast and agile. If you want to look like them and perform like them, slogging away for miles on end on the treadmill will not do the trick. Explosive movements like the power clean, snatches and sprints, coupled with squats and bench press will steer you more in that direction.
Comparatively look at an elite Olympian like Michael Phelps, the greatest Olympian of all time by many regards with a record 23 Gold medals. Michael stands around 6'4'' and weighs around 194 lbs. He's got about three inches on the NFL defensive back and weighs about the same. He is long and lean, built for endurance, doing both sprints and long distance work in the pool. Everything he does in the weight room models and supports what he does in the pool.
The type of exercise you choose will allow you to pick your physique (as much as genetics will allow). Understand that there's more than one way to be fit and healthy. Sometimes it's sprinting, sometimes it's swimming or running for distance, and other times its lifting super heavy weights.