Do whichever you like, but consider the following:

1. As far as I can tell, you get 1 (one) blog. If you think you might want to blog about a particular subject later, you might want to save your blog space for that.
2. There is currently a lot more traffic in the forums than there is on the blogs. That may change in the future as more people begin blogging but if the comments of others are important to you on your journey, you may want to consider putting your journal where you'll get the most.
3. There is a sub-forum titled "Fitness" which would be the perfect place for your journal if you choose to place it in the forums.

Best of luck to you!