We've known each other through the internet and our first date is happening in the next few days, I've figured out lately that she knows a lot of guys with good body building (as it clear from their profile pictures on Facebook) and she likes their pictures where sometimes their muscles are shown, and I'm kinda chubby right now, not fit at all, which makes me uncomfortable and I'm afraid that she won't eventually like me.

You didn't really ask a question, but as far as her liking you, you'll just have to wait and see...physical attraction is a component to a relationship for most people, but that doesn't mean you have to look like an Adonis...

If you don't like your body, work on changing it....it won't help you with your upcoming date, but for the future it makes a big difference if a person is confident (and people who worry about their own appearance, are typically not all that confident)...