I think the 18/55 marathon plan is his lowest mileage plan, though I think he also throws a sop to those running less than 40 mpw. Take a look at his 18/85 plan for something comparable to his HM plan. He does have multiple plans for the HM in his older book. Most of his plans in the older book are also easily modifiable. As long as you hit the quality workouts, the other days are pretty much up to what the individual wants or can do.

Agreed.  That is his lowest.  He also has the 12/55, 18/70, 12/70 and the ones peaking at more than 85.