This question is easily answered with "eating habits". You can workout all you want and leave the gym and begin chowing down on bad foods and reverse any internal gains you may have made. In reality you can be overweight but fairly physically fit just not chemically fit. In other words you may delay the onset of disease such as cardiovascular risk with exercise but you won't maximize the potential outcome if you eat poorly. In addition you won't get the most out of your workouts.
To avoid this, follow some simple rules:
Must eat foods:
Fats
Fish oil, almonds, pecans, walnuts - RAW; salmon; olive oil
Green Tea
Brewed - no iced tea in a can or bottle
Vegetables
The more the better - keep it very colorful or eat very dark leaves
Fruits
Mix varieties frequently - Dark red or purple skin are excellent
Whole Grains
Brown, wild whole grain and those that have at least 3g of fiber per serving
Water
Plain, no artificially sweetened water or juices
Lean Protein
Baked or grilled, not fried and eat it with every meal
Lastly, to preserve the neuroendocrine boost to your metabolism post workout, avoid fats for several hours. Studies show that the ingestion of fats post workout, blunts the increased hormonal affects which helps to repair and grow lean muscle mass and increase fat metabolism.