I can't work out how to do in Purple Pen is adjust the length of courses, eg for a sprint course where the distance is meant to be shortest possible route, rather than straight line.
This isn't currently possible in Purple Pen.
There is a list for support, suggestions and general bug-reporting - see this mailing list. Someone asked the same question in the past - see http://groups.yahoo.com/group/purplepen/message/9 for example. It's probably worth asking the author (via the list) about implementing this in the new 2.0 release.
Workaround - maybe specify the shortest length in the "Class list/secondary title" part of the descriptions if it's critical.