I find it interesting that so many "more mature" players think that their positional chess is better than when they were younger. I feel the same way, though sometimes I wonder if I'm just deluding myself. I definitely had a smaller toolbox (oo er!) when I was in my 20s, but perhaps I was more proficient at using them. Nowadays my head is full of half-understood positional ideas and tactical themes and I often try to shoehorn them into the game when the position doesn't really merit that particular approach.
As for getting tired after playing long games - my approach for the last couple of seasons has been to play much quicker, often spending just a minute a move for the first twenty moves. This means that I often have no discernible positional advantage, but my brain is still fresh and the advantage on the clocks can lead to blunders from my opponents.
Thinking about Kasparov's crushing victory over Short again, I realised that they are almost the same age (Short is 50 in June). I wonder if he would have beaten a much younger player (David Howell say) as easily?