Each year since 2006, they have gradually trekked the length of New Zealand, through private land, from Cape Reinga.
Next year, they will complete the journey, ending in Bluff.
As administrator, she's arguably the driving force behind the event.
"I am just the captain that steers the ship and I am very grateful for the support of the other trustees and the hundreds of volunteers over the years that have given their time.
"It really is about teamwork and having the passion to help others. This event is very unique and you have to be a bit of a MacGyver type person with a can-do attitude."
The originator of the GNZ Trek, Steve Old, concluded his involvement after the first three years, at which point Johnson established The Great New Zealand Trek Charitable Trust to continue the event.
She also used $15,000 of her own money to ensure the organisation of the event carried on that year.
Before being involved with the GNZ Trek, Johnson did not have a personal connection with MS but now has been touched in more ways than one.
Perhaps most significantly is in 2012, when the owner of a farm she wanted to use for the trek happened to have MS.
"Honestly, we just sat there for about half an hour and just cried because she didn't realise that there was people out there like us doing things to try and help people like her.
"I think of all the treks that I've done over the 13 years that is probably what sits in my heart the most and makes it all very worth while that you are hopefully going to help people one day like her."
As the concept, along with Johnson's involvement, comes to an end in 2019, it seems fitting she has been "humbled" by such an accolade.
"I will be 65 and 14 years is a long time. You do make sacrifices ... there's things that Hepa and I both wanted to do together that isn't going to happen now. But it is definitely worth the sacrifice.
"We have met some amazing people and we've been to some amazing places. It is a great way to actually see the real New Zealand, not just the commercial side of it."
To date, the GNZ Trek has donated $316,000 to the Malaghan Institute for pilot research into MS.
Further donations of $10,000 to the Christchurch MS Society and $56,000 to the Wellington MS Society have also been made.