Unlike the poster above me, I am going to give some useful tips other than try to be a smartass.
First off, I would like to give you props for doing this. Personally I would be horrible at commentary, so great attempt. Allthough your speaking has many flaws:
1: Know what you're going to say before you record. That doesn't necesarily mean you need to make a script and read it in monotone, it more means that you need to write down a few notes about, for example, names of DLC, cars, statistics and other main points you want to adress in the video. This will keep you from going "uhhh" or "ummm" and stuttering alot.
2: Don't ramble. If you are going to go off topic, make your point and go back to your main topic.
3: Make a connection between your commentary and the video. While you did talk about route X and other people in the room, there was really no connection between what was happening on the track and what you were saying. Even if your topic is to talk about DLC, you should still be commentating from time about the race and not disregarding what is going on on the track. Maybe talk about what cars are better than others on the track, or how they should be tuned, or maybe do some race commentary. There are many possibilities.
4: Emphasize your voice. Your voice really seems to tail off at the end of sentences (especially the rambling ones) which makes it hard to understand and makes you lose your audience.
5: Make sure the topic is intriguing
6: Keep making videos and practicing. Practice makes perfect! 👍
Take a look at this GT5 commentary. It is very good. I am going to link to this video so it doesn't steal the thread.
http://www.youtube.com/watch?v=5wJ66YsuFV8&feature=youtube_gdata_player