Myth
If you exercise for longer, you will achieve better results.
Fact
It is not the duration of your workout that determines the results you achieve, it is the intensity. You will feel and see more improvement in a shorter period, by increasing the intensity level of your workout, rather than extending the length your training session.
Very true, I train at 80 to 90% of my maximum hear( rate for around 45 minutes and achieve excellent results..