1) I'm a chatty runner. I'm slow and tend to be towards the back where a lot of runners appreciate distraction or need some positive motivation. However, if someone didn't want to talk to me then I would not be offended if they politely said that they weren't in the mood for talking and I would leave them alone. It is usually quite easy to spot those who don't want to talk. Most slow runners tend to naturally congragate into groups, even if they don't know the other members of the group. Someone chosing to run on their own is quite obvious and I would tend to leave them alone. Don't forget that even chatty runners have times when they want to be left alone with their own thoughts and I respect that. You don't know the character of the other runners, or whether they have something going on personal to them that they don't want you interfering with. What I am saying is that it is not uncommon for some runners to want to be left alone
2) Coming last. Yes you might, but it is extremely unlikely. There is no shame in being last. I've come last in half a dozen races and it has always been a positive experience with lots of cheers and support in the final stages of the race. There is no reason to fear being last. You are still better than all those people who ccouldn't be bothered to enter because they didn't think they were good enough or were too lazy.
3) I'm not competative and the reason that I race is because i like the atmosphere, I like to run different routes, and it is nice for someone else to plan the route for me occassionally. No chance of getting lost, feed stations provided for you and the bonus of a goodie bag at the end. Its personal preference and iff you don't want to race then you don't ahve to, but I would give it a go at least once so that you know what you are missing out on.