The '1 in 5' chance only correct when applied to the number of people entering the ballot compared to the numbers who get a place.
ie 25,000 places and 125,000 are in the ballot.
But I was led to believe that the draw is not done on a truly random basis - I am pretty sure they take age, sex and estimated finish time into account to try to ensure a 'balanced' field. What that means is that if you are unfortunate enough to fall into a heavily oversubscribed category e.g. V40 Male est. 3.45, then your changes are significantly less than 1 in 5.
I am pondering entering this year, as my first marathon, but not looking to commit until April/May time. But am a bit worried that the cancellation of the Liverpool Marathon and the fact that the race was voted the UK's fav might mean its a sell out before then.