So, the 32-year-old decided to live stream his whole backyard journey to prove he's a man of his word ... while asking viewers to donate to his fundraising campaign for COVID-19 relief throughout the event.
The #6MetreGardenMarathon took 5 and a half hours to complete ... and after word spread about his accomplishment, Campbell raised more than $33,000 for his cause!!
FYI -- Campbell is a damn good javelin thrower too ... he was once the 8th best junior thrower in the world.
"There’s not much positive news around at the moment," Campbell said on the "TODAY" show. "To take people’s mind off of it for a day and give them a bit of a laugh and a bit of entertainment, that job's done."
He added ... "It’s just been a very long week at home on my own."
"As soon as you gain momentum, it was like, if I’m going to do something like this, then it’s got to be for a good cause."
And, if you have nothing better to do, you can watch the whole marathon below.