An excellent question. You have undoubtedly seen the leaders in televised triathlons chatting with the cyclists, the TV cameramen, and the commentators. You have probably also seen them giving high-fives to the spectators, waving flags as they approach the finish, and slowing to a jog or full stop so that they can look back and admire the distance that they have put on the rest of the field.
We, however, are runners, not triathletes. We're better than they are. We understand that there is a fine line between enjoying the lead and belittling the competition.
The proper demeanor while leading a marathon is a look of concentrated intensity, which neither minimizes the competition nor the task at hand.