Totally understand where you are coming from Reflector; in fact I asked a similar question on this very forum a few weeks ago because I was (and still incredibly am) paranoid about how slow I am on the bike when gents in very tight lycra go whizzing by.
I ultimately found my solution in the suggestions above - find the results table for one of the larger races, drag the data into Excel and average out the times and you will get an idea as to where you need to be.
Don't be discouraged if you find, like me, you have a long way to go!