I don't use it anymore as you can just adjust the laps in the season file yourself. My big problem with RaceLengthEditor.exe is that it lists races by the track shortname which doesn't always make it easy to know what track it is. For example "British GP" is Silverstone, but how do you know it isn't Brands, Aintree, Lulsgate or even the fantasy track Dragon's Nest all of which have "British GP" included in their name in the Track Database listings.
To manually set the distance, everything is based on the GP distance laps which is what the numberOfLaps=xx setting is for each track listing in the season file.
Novice is 7% of that value
Short is 15%
Long is 30%
GP is 100%
So if you set numberOfLaps=100 and select a novice race, you'll have a 7 lap race etc.