Remember these key principles for success in your exercise:

• There is no good or bad exercise. But there is the right exercise for the right person, at the right time and performed the right way.
• Exercise is a stress and every session has a cost. Balance your training with the right amount of quality recovery to ensure the effects are positive.
• Fitness is a science. Being fit to run a marathon is very different from that required to sprint 100m or surf serious waves. The type of training you do depends on your specific goals. 
•  Like the saying goes if you fail to plan you're planning to fail. Get clear about your goals and set objectives for your training.