First, it is silly to think that someone running as fast as they can would bring the same distress to slower runners as running up the score since the point of racing is to run as fast as you can but running up the score is not the point of team match.
No. You have not offered any evidence it is 'silly' to think that, nor have you offered any evidence that athletes do not, in fact, think that.
If anything, it seems to me it could be more distressing, not less. The loss by a team sports athlete can be partially externalised as the 'fault' of other members of the team, but there isn't any such externalisation that a runner could make. If somebody beats you commandingly, it's because you personally were not fast enough.
Second, I have made no claim about a moral difference. Being an asshole is not immoral. Why you keep babbling about "moral differences" is truly a mystery.
I am not babbling. You think one situation requires you to act in a certain way in order not to be an asshole, and another requires no such obligation.
Third, I have made no claim about the obligations of team sports players. I pointed out - and many others have confirmed - that running up the score is considered an asshole thing to do because having the largest score differential is not the point of a competitive match. Nor is running up the score against a hapless opponent an example of doing one's utmost, because there is, in essence (especially in this instance) no effective opposition.
Failing to play as effectively as you can out of pity for the other team is not doing your utmost at playing. It is compromising your utmost to serve another goal.