Noomi.:.GWDT
Tumblr has a huge fitness community, but it isn't really conducive to actual discussion. If you want feedback/comments/etc, I'd go with Wordpress. Some Facebook pages also do well (for example, I follow two gals on there under "The Sweaty Betties" and they just post daily statuses- HUGE following, they can sell merch because they've done so well that people love 'em).