Originally Posted by Wee G

Just avoid moderate to long duration at moderately elevated HR....eg running for an hour. Either do moderate duration low intensity (eg Tom and his 40mins fast walking) if getting / keeping lean whilst building LBM is the goal, or do some very high intensity short duration stuff (high intensity interval training (HIIT) or Tabata work if maintaining / increasing conditioning whilst gaining LBM is the goal. If doing the latter total work time should be less than 20 minutes.