Bexley MP James Brokenshire has told how he set himself milestones including a 22-mile walk in memory of murdered teenager Jimmy Mizen to help him recover from lung cancer surgery.

The 50-year-old, who stepped down as Northern Ireland Secretary after his diagnosis in January, is set to cross 21 of London's bridges on Saturday to complete the challenge.

The MP for Old Bexley and Sidcup made a swift return to the Cabinet when he was appointed Secretary of State for Housing, Communities and Local Government last month.

He is one of an estimated 300 people, including Barry and Margaret Mizen, who are hoping to raise £50,000 for their charity For Jimmy - set up after the murder of their 16-year-old son Jimmy 10 years ago - on a walk between Richmond and Tower Bridge.

Jimmy bled to death in a bakery in Lee on May 10 2008, after violent teenager Jake Fahri threw an oven dish at him.

Mr Brokenshire said: "At the start of the year, when I had my diagnosis, going in for surgery to remove the top lobe of my right lung, I certainly didn't think then that I'd be fit and well and able and doing a 22-mile walk.

"At that time, doing a one-mile walk seemed like an incredible challenge.

"But I've just been very blessed in picking up my tumour as early as I did, having successful surgery, having bounced back as quickly as I have done, and the incredible care and support I received from the NHS was outstanding and just very humbling."

Mr Brokenshire said he had "some pretty dark moments" when his health deteriorated at the end of last year, but the decision to resign from the Cabinet was an easy one.

He said: "It's one of those moments where it was just very clear to me what I needed to do. I needed to step down, I needed to focus on my health."

The father-of-three said he has returned to the Government with a "renewed zeal" to get the job done in his new post.

"I had my surgery on January 17 and to effectively come back at the end of April, I never anticipated anything like that," said Mr Brokenshire, who credits his early diagnosis after visiting his GP.

"If I hadn't done that the outcome could have been very, very different.

"At the start of the year I was thinking about whether I had a future, let alone a future back in government."

Mr Brokenshire said the 21 bridges challenge, along with a walking trip to the Scottish Highlands, has helped his recovery.

"I think it's having these milestones for me has actually helped me on my recovery, to actually set me on those goals and of course it's really important for me to support this incredible cause, but equally for me personally on proving to myself that my fitness is back," he said.

"So to be able to do that by supporting such an amazing family and such an amazing cause combines everything together."

Mr Brokenshire, who first met the "humbling and inspiring" Mizens nine years ago, said the annual walk was especially poignant this year, coming on the tenth anniversary of Jimmy's killing.