Already 1695km and 40 days into his run, Mr Brown (39), an Australian and Japan-based plumber, translator and builder of ski and snowboard courses, is feeling "refreshed", despite running an average of 50km most days.
He has visited schools, met mayors, become a media darling and made dozens of new friends along the way.
Mr Brown visited Japan on a school exchange as a pupil at Mount Aspiring College, before living there on and off for the following 18 years in the Tohoku area, which was badly affected by the 2011 earthquake and tsunami.
His run is to raise money for the country's disaster-hit areas.
He expects to complete the feat on July 8, nearly 100 days after setting out on April 1, after running up to Sendai in the middle of the tsunami-hit area, then driving to the north and running back down to Sendai.
"After the way that my New Zealand run [a Stroke Foundation fundraiser] went, I was on a high and thought I can ... see if I can do it in Japan."
He had a simple method in mind for promoting his cause.
"If everyone I met can tell one person about what I'm doing, then maybe they will talk and it will spread into something special, and right from the start it worked.
"I arrived and within two days I had three of four islands almost sorted with homestays [accommodation] and people to meet with.
"I got on TV, radio and in the newspapers. It was amazing," Mr Brown said.
A compression fracture in his spine as a result of running while pushing his gear-filled buggy slowed Mr Brown down temporarily, but things improved after two weeks of walking, followed by transferring his buggy to the support crew's van.
After Mr Brown's sister paid for their Wanaka-based father Ron's ticket to Japan, he joined his son in Japan for a week last month and drove the support van.
"For me, it was perfect timing, as I was pretty down from having to walk for two weeks ...
"I was alone all the time and it just gave me a boost and a little goal to get to, the day he would arrive."
Mr Brown's next highlight will be climbing Mt Fuji on May 18.
