Remember these key principles for success in your exercise:

• There is no good or bad exercise. Rather it 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 and quality of recovery to ensure the effects are positive.
• Fitness is made up of many factors. The fitness to run a marathon is very different from that required to sprint 100m, throw a discus compete as a bodybuilder. The type of training that you should do depends on your particular goals. 
•  Like the saying goes if you fail to plan you are planning to fail. Get clear about your goals and set objectives for your training.