You show up on an old steel bike with fenders, out of shape, you were right on your feeling that you were not wanted.If you were in shape, would you be able to keep up with them? Who knows? You could be so much slower than them it is beyond belief. Or you might keep up. Start with a slow group and if you're faster move up. Nothing worse than an out of shape slow guy dragging down the group.

Your fitness will matter most, the fenders and rack won't make to much difference.