I have just got back from a run and wondered, should I exercise? My knees hurt, my muscles ache, I am out of breath. Is exercise really that good for me or should I listen to others who say "we are only given a certain number of breaths so don't waste them exercising". I do note that those who say this are usually overweight!
There is no doubt that we do need to exercise, science has proven this. One thing that I have found with exercise, particularly running or cycling, is that it clears the mind and allows me to concentrate on things other than work. Furthermore, a good way to find an answer to a problem that you are struggling with is to go for a walk and don't think about your problem. Chances are you will have an answer by the time you have finished walking.
Exercise gets your heart pumping oxygenated blood to all parts of your body and in particular, to your brain. This allows the brain to work better. Exercise also produces endorphins, the natural 'feel good' drug.
Exercise need be no more than a 30 minute walk each day. If sweating in the gym is your thing, great. Just ensure that you are doing an exercise that you enjoy, that way it doesn't seem like such a chore.