The Yangebup woman's mum died in 2003 after a battle with pancreatic cancer.
Twelve years on Ms Johnson will be among the thousands taking part in the HBF Run for a Reason this Sunday.
So far she has raised more than $500, which she will donate to the Cancer Council WA.
Get in front of tomorrow's news for FREE
Journalism for the curious Australian across politics, business, culture and opinion.
READ NOWShe had set a target of $200.
Ms Johnson said she now felt strong enough to give something back.
But a personal mission to lose weight and get fit has helped, too.
"It's taken a while for me to reach that point where I can help others, but it's my turn to give something back," she said.
"I can't wait to do my mum proud and am so excited to have raised so much money and hopefully I can raise some more. I've been overwhelmed by the support shown."