Exersize is one of the most important things you can do for yourself. I think it's a major factor in keeping not only physical well being but mentally as well. I would much rather run or walk outside than do it on a treadmill or indoor track. When you are outside your level of attention is higher to avoid curbs, sticks, cars, etc... Dealing with the weather can be a factor as well, be it 100 degree heat, cold rain or ice. It just makes it more interesting and challenging to me.
I used to race bikes when I was younger and the best thing about that sport was the eating. You burned so many calories that when you wern't riding you were constantly eating (anything you wanted) and never a worried about gaining weight. Heck, even while you were riding you had to eat to keep from bonking!