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Don't worry, if you're not proper/fast/whatever then you'll find out fairly soon. But you'll at least be further ahead than you were before.
It sounds like I'm a similar pace to you, and I do the same - sit somewhere in the middle and jog off politely, then pass people gradually while watching the front runners disappear into the distance. I could probably do with being more aggressive/assertive/bettererer too.
I'll give it a go next time. Be nice to go sub 20. I guess I think of the front being for the proper/fast/whatever runners and I still don't really think of myself as a proper/fast/whatever runner.