I think that cyclists might do that, but it isn't necessarily optimal training. When I was cycling more (on road) I was advised to spend a lot of time (read: up to 4+ hours at a time) at <70% of my max heart rate, then once I was conditioned, include short bursts of speed at rates above my lactic threshold. There was a notion from more experienced cyclists that what you're trying to do (which is very common) is simultaneously too much and too little. Its too intense to train your aerobic capacity, but also too moderate to really train your lactic threshold, V02 Max, or max leg strength.
That being said, that's only comparing it to an optimal training schedule for experienced road bikers. If you're having fun and feel like you're getting better, keep doing it.
I would recommend that you try and be aware of signs of overtraining - if you feel like you're not recovering between rides, or you're starting to pick up aches and pains that feel like their not going away before its time to ride again, or if you just find that you're getting slower and less explosive, you may want to reduce your pace or weekly mileage.
I'd recommend that you research beginner training plans for cyclists. I know that Bicycling Magazine published a nice small book for beginning road bikers, and they may have a good one for mountain bikers too. You'll get a lot better if you're following an established training plan than if you just go out and beat yourself up. :)
EDIT - I can't speak to the potential long term negative effects of keeping your heart rate that high for that long. Whether it exists at all, or to what extent it does / doesn't matter to people of different ages. That's outside the scope of the training literature I read, or my experiences of those of the other (20-something) cyclists I used to know. The only thing I could say is that if you're feeling short term symptoms of overtraining, your odds of doing something bad are likely higher.