The Nakahechi route can be walked with multi-day trips or shorter walks. One popular shorter walk is the Hosshinmon-oji to Kumano Hongu Taisha section. It is a seven kilometer walk with a mixture of isolated villages and forests, and finishes at the Kumano Hongu Taisha Grand Shrine. There are some scenic lookouts along the way.
Being based in one of the hot springs in Hongu is an excellent choice, such as Yunomine Onsen or Kawayu Onsen. Yunomine Onsen is along the Kumano Kodo route. Some of these accommodations at Kawayu Onsen have free shuttle busses to the Hosshinmon-oji trail head, such as Fujiya Ryokan.
As mentioned in the previous answer, public busses run to Yunomine Onsen and Kawayu Onsen from Tanabe in the west and from Shingu in the south. There is also a very long bus route to the hot springs from the north through Nara prefecture.
The local community based government run reservation system called Kumano Travel has a list of accommodations for the above hot springs. They really helped me out with my trip to the area.
This is one of the best ways to stay along the Kumano Kodo Nakahechi pilgrimage route being based in a ryokan with some moderate walking routes nearby.