Couldn't agree more with the above! Bet when you get there it'll be YOU looking around at everyone else, not them looking at you! I've just started going, and was plodding along on the treadmill to warm up, but nearly fell over when I couldn't stop peeking at the guy next door, headphones on, towel round neck, running flat out, with a bead of sweat hanging off the end of his nose! I was mesmerised - when was it going to drip? He was completely oblivious, both to me and the sweat!
As everyone else has said, other people are completely wrapped up in themselves, especially the athletic ones. But I know just how you feel, I was exactly the same, so pluck up the courage and make your first visit - you really won't regret it. Have fun...