Unlike marathon training, your HM long run will be longer in duration than the goal race. And the pace will be far from the goal pace. So it is in no way a specific workout, but still useful for general endurance. Hence your limiting factor in the 2-3 months before the goal race would be recovery time so that you can nail the more specific workouts. 'All the way up to 20' might be from base building phase of an athlete who likes to go long and takes this kind of run well. Does anyone running HM at high level really do that much peaking for a HM?
It can be argued that an easy 32K long run wouldn't be specific for marathon either, but it's the only type of run when you spend about the race duration running, and if you push the pace just a little it can be like 80% of marathon effort, so really much closer.