How much training is too much?

I've blogged quite often on the subject of overtraining and the duration of training sessions.
Today I've read this post from the always interesting Blue Mountain Fitness website.
I highly recommend every endurance athlete to give it a read and to consider a few things.
I think we all agree that having an active lifestyle is great. It's great to feel that your body is full-functional and that your efforts bring better performance, but in my opinion the best result we can obtain from training is being able to keep ourselves active for as long as possible.
So what's the point of looking for always-higher endurance performances when this could translate in having to give up with training at an early age? I am not considering only tragic events here, but also the natural wornout of knees and ankles for example.
Anyway, according the recent studies linked in the article it seems that my usual training routine isn't that bad.
I do daily woukouts, every day I try to vary intensity and the type of activity, but I never exceed the 50 minutes. My usual workout is between 45 and 50 minutes, be it footbiking, hitting the heavybag, bodyweight training, fast walking, playing basketball. When I'm doing something where I want to improve my performance, like ifootbiking, I try to add intensity and to enhance the mileage I cover in those same 45-50 minutes.
Good to know that I'm doing this right!