I’m not sure I’ve wrapped my head around what you’re asking, exactly, but the meat of it is, how to stop being negative?
If so, there are a couple tactics. There’s a great book out there called “The Mental Game Plan: How to get Psyched for Sport” that details a lot of ways to improve your self-image. Alas, a book is no good if you don’t put it into practice.
Why is it that you feel you have a negative attitude? How does it manifest itself, for you? What’s the “mentalese” that goes through your mind that determines your attitude is good or bad (surely you’ve experienced times of both, as everyone does).
As far as the mentally tough vs. physical preparation bit, I feel I’m miles ahead of myself in mental preparation vs. the physical aspect. I wouldn’t say that makes me tough, just that I have a rough approximation of what this is going to call for and am willing to mold my mind into whatever shape it takes to make this a successful experience. In the heat of the run, I find that keeping my mind as clear as possible is helpful. If I’m not thinking anything in particular, I can’t have a negative attitude as attitudes are born from thoughts. That’s the best approach I can take right now because, by nature, I’m a pessimistic person. I never think I measure up and find it difficult to accept praise for anything (on and off the road) because I have a sense of how far I have left to go. Trying to think only positive thoughts hasn’t worked for me, to date, and it’s as clear in my running as it is in any other aspect. The best I can do right now is minimize or eliminate the negative thoughts and just try to enjoy each run for what it is, as opposed to what I want it to be eventually.
I’m not sure if this is the kind of answer you were looking for or not, my apologies if it isn’t.