Matt Z wrote:
For example, what factors might effect the optimal duration of a workout? Do individual factors like age and level of training change the equation?
Age definitely plays a factor in workout duration. Recent studies indicates that cortisol levels rise markedly in trainers over 40 after 60 - 75 minutes of moderate to heavy exercise. They recommend drinking carbohydrates (sports drinks and the like) to blunt the rise in cortisol. The cortisol levels also rose in younger trainers, but not nearly as much.
But in both cases, testosterone and growth hormone output dropped sharply after 60 - 75 minutes. And since the hormonal environment mediates potential muscle growth, that is significant
If you enjoy working out, by all means, go for it. It beats watching TV. But be aware that at best you might be engaged in needless effort and at worst you could be increasing your likelihood of injury