Personally I have never bothered about running the distance beforehand, as long as I could run 80% distance then that was fine. I concentrate on the speed side, as on race day you would hopefully start off slightly slower than normal anyway, which would also assist you in that last 20%. There are also no rules to say you have to run all the way either, just enjoy it, and if you go off too fast it very likely wont be as enjoyable.
Best wishes, NN