In America, more is more. And I think we have started applying that to our training too: "go hard or go home" is the new fitness trend. I think that we know, without a doubt, that intensity is the most effective method of creating physical adaptations. And it has the added benefit of being wildly efficient. But I think there are two aspects of intensity that are overlooked: first of all, it's relative, and second of all, it's intense (duh).
Intensity is relative--which means that a professional athlete and a couch potato can experience that same intensity doing entirely different things. While there's a push to go harder and harder, what we sometimes miss is that you might be working at the correct intensity for a given day. Even if you aren't doing 300 million back flips then deadlifting 300 lbs. 5 pushups on your knees can be high intensity if it matches your fitness level. Yes, we want things to be challenging, but know that if you push too far, you are certainly doing more harm than good. It does not help to compare yourself. Yes, people are stronger than each other, but people are also taller, heavier, and a many other things that can contribute to the difficulty of an exercise. If you are tall, pushups are harder. The lever is longer. It's physics. Learn to monitor your own intensity rather than use others as a gauge.
Intensity is also intense. We don't live our lives at 100% intensity--we aren't always wildly happy or sad, we don't expect ourselves to work hours and hours without sleep (or we shouldn't). Intensity is so useful and effective, but when it is constant, it takes it's toll. Stress is a great example. Stress is amazing because it activates our fight or flight response and gives us the hormonal firepower to deal with the extreme challenges of life. However, if we are in a constant state of stress (which many of us are), it leads to extreme health problems. We are not meant to be in fight or flight constantly, and those elevated hormones lead to chronic issues. Intensity is the same. When the volume is managed, intensity is the most effective way to train. But if that volume reaches a certain point, it means the efficacy is compromised and you are harming rather than hurting. Intensity is effective and efficient. Efficient means you don't need to overdo it.
How much is too much? That depends. I think we can handle a lot more than we think. Usually. The best way I have ever found to manage intensity is to see how I feel on a given day. Some days I'm ready to work. Other days I am not. And on the days I am not, I don't try and force myself. I pick something less intense and do that. I still train, but I focus on technique or stretching or just slowing everything down. Learn to adjust your training to how you feel. If you are stiff, you might need to come to class early to foam roll. If you didn't sleep, you probably should skip the 8 mile run and take a nap. You are the only one who knows what you are feeling, and you must learn how to manage your training based on your current state.