My suggestion is to contact the heavies around where you live and get them to break your leg the day before the race.
Failing that- a 20 kilometer race is quite short so get yourself nice and warm before the start- get the heart rate up to 75% of your max about 10 minutes before the off and then just keep warm.
You will have to push right from the start so if you are up to speed at the start- without being worn out- then you will be able to push from the off. Rather than only starting to get into the groove 5 miles in. You may struggle for the last 19 Kms. but at least you started right
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How long was I in the army? Five foot seven.
Spike Milligan