It's a tricky and clever-sounding statement, a half-joke because it's phrased in a surprising way that stops you and makes you think. The meaning is: "I planned to reach a destination. I've come part of the way. I'm not going to stop here. I'm determined to go on and reach my destination." These would be straightforward statements: "I didn't come this far to turn back now." "I didn't come this far just to see the mountain, I came here to climb it and I'm going to." The tricky statement is correct, but some implied words have been left out. A plainer statement would be: "I didn't come this far in order to only get this far." That is to say, "My goal was not merely to get this far." Without more context, I can't guess why the speaker used this phrase. OK, I just Googled. It seems to be come kind of catchphrase that's becoming popular. Lots of celebrities seem to be using it. It's slightly clever or joking precisely because the second phrase is unexpected, and the repetition makes you stop and think for an instant. Obviously, people hear it, think it's cool, and then use it themselves.

But who is the quote credited too

It's a matter of taste, and I must confess I don't find it clever at all. On the contrary, it shows a low language ability. It is much better English to say, "I didn't come this far to stop now."

Reece, thank you very much indeed! It's absolutely clear now.

It is a clever way of saying 'I didn't come this far to stop now'. For example, if you were running a marathon (26 miles) and you had run 20 miles, you might say I didn't come this far to only come this far. Basically, it is saying, you have come a long way (or achieved a lot) so there is no point stopping as you are close to the end. You have set out to achieve something, and have come most of the way, so you are not going to stop now.