It's all relative to the individual, Daniel, and the best way to gauge if we're doing too much is how we're feeling. When I started I found out I was doing too much when my shins started hurting, so I did a lot less for a couple of weeks and then built it up.
I try to work on 3 week cycles, where I'll have a hard week, followed by a harder week, followed by a rest week where I'll run about 50% of the distance covered in my hard week.
I try to build up by 10% per week too (still resting every third week). I'm middle-aged, and I need the rest week.
Maybe the level you're training at is fine, but you just need a rest week?