Well, the older you get the more you have to pace yourself. You pick your spots to go all out, and coast a bit when you deem it to be o.k.
The problem with the Celts is that there are three older guys playing a majority of the minutes. So, at most times, there are 3 out of 5 guys pacing themselves. This gives other teams a chance to seize the momentum and turn the game around.
The minutes of the older guys need to be reduced to that point where they can go all out the whole time on the court.