The church has made much of this disqualification spoken of by Paul, and yet the result of their teaching is the very thing that Paul was referring to. Here's what I see in it.

And I do all things for the sake of the gospel, that I may become a fellow partaker of it. Do you not know that those who run in a race all run, but only one receives the prize? Run in such a way that you may win. And everyone who competes in the games exercises self-control in all things. They then do it to receive a perishable wreath, but we an imperishable. Therefore I run in such a way, as not without aim; I box in such a way, as not beating the air; but I buffet my body and make it my slave, lest possibly, after I have preached to others, I myself should be disqualified. 1 Corinthians 9:23-27

The most obvious thing Paul spoke of in the race was the FOCUS of the runner. All of his attention was set on the goal. He could not turn around, sit down, listen to those calling from the sides or he would disqualify himself from the race. Paul used this to describe seeing our life in Christ as being found only in Christ. The obviousness of the picture to the Corinthians was that the wisdom of the world was calling to them to keep their attention off of Christ. They had been deceived into thinking that they were pursuing spirituality ... godliness, but the world had side-tracked them into the craziness of what they had been experiencing. By Paul's bluntness he made it clear that the activity of preaching (which is all the other teachers offered) held no magic in spirituality.

if after preaching to others, I myself am disqualified... 1 Corinthians 9:27

Preaching Christ was a joy of Paul's life, but it was not his life. Christ was his life. So, in this, Paul touches on a couple principle points of the race.

Do you not know that those who run in a race all run, but only one receives the prize? Run in such a way that you may win. 1 Corinthians 9:13

What's he talking about? It's not competition against each other, for that is against everything he taught. He's talking about the secret of those who win a race: focus on the prize is what makes the difference. Pretty simple really, especially when you also consider the Philippians passage about the race. He was telling them simply, that FOR HIM, it was about he and Christ. And for them, it needed to be about each one and Christ. It's not about who is watching from the sidelines, it's about the goal ... who, for us, is Christ. We are to run as if He is all that matters (running as to win). After all, if we were all running the race side by side, then only one of us would win ... not very good odds ... and this would only instigate heated competition, which is what they were doing through the wisdom of the world. No, Paul was referring to the simple reality of the focus of one who knows that only through focusing on the goal can he win. It's not questioning our destiny in any way ... it's about the eternal, right-now life. For when I am seeing Christ I am seeing myself truly! And I am also seeing YOU truly! You had also asked about:

...by their fruits you shall know them Matthew 7:20

Guess who Jesus was talking about? The fakers. The pretenders. The reason He mentioned them was that they presented this facade of spirituality and of righteousness. But a bad root produces bad fruit. Eventually, the pretenders are shown for what they are ... so don't get fooled. Now, there is a reality that a good tree produces good fruit. But examination of such classifies it in the same category of things that can be compared, so you don't see it that way ... BUT YOU CAN SEE IT! You see it in the fact that it doesn't speak of itself, nor is it interested in offering proof. It is totally backwards in how you would see it. You listen for simple life being expressed. Those who make all the noise about spirituality are the ones you begin to question simply because they are making so much noise about it ... you begin wondering who they are trying to convince ... you, themselves, or both? The one who questions himself ... even who questions his own salvation ... you begin to sense the sound of a child who wonders if Daddy still loves him, and NOT the sound of the one who is denied by God. Backwards! Then, again ... everything Jesus did was backwards and didn't fit with what we thought it should.

...even though we have know Christ according to the flesh, yet now we know Him thus no longer 2 Corinthians 5:16