He will attempt the 13.1 mile course around the capital city after running to the summit of Snowdon last Saturday and completing an 11,000 feet sky dive the previous weekend.
Hayward was diagnosed with ocular melanoma at the end of 2012 following a routine eye test, the only way of detecting the rare disease before the risk of it spreading and becoming deadly increases dramatically.
He was treated at the Royal Liverpool University Hospital at the start of the year and is hopeful that the early detection and radiotherapy was enough to prevent further complications. However, the former Ebbw Vale, Gloucester and Sale Sharks pivot decided to embark on this triple personal challenge in order to raise awareness of the cancer that could affect anyone.
He said, "This disease is rare but deadly because the only way it can be spotted in the early stages is through eye tests. My condition was detected last July through a routine eye test at Specsavers in Ebbw Vale resulting in an early diagnosis which will hopefully mean the tumour can be contained through radiotherapy treatment.
"So my strong advice to everyone is to get your eyes checked out. Some people survive Ocular Melanoma but lose their sight through late detection, while for others it can be more complicated if the disease spreads to other organs. It can affect anyone aged 8 to 80 and it is not as a result of my sporting background - it's one of the few forms of cancer where lifestyle has no impact, it can affect anyone.
"I cannot put into words how grateful I am for so much support I've received since deciding to publicise my situation and complete these challenges. After the experience of the past year or so I was determined to do something to raise awareness of the potentially lethal disease. If one person survives through our efforts then surely it's worth our time, effort and cash.
"I managed to summit Snowdon in 1 hour 14 minutes last weekend. It was very tough with strong winds but I was running with lot of emotion and that helped. I'm really looking forward to the Cardiff Half now. My legs are pretty tired from the training and Snowdon but I'm hoping the crowds and the atmosphere will make a big lift on the day. We have managed to raise £5000 already so it would be really great to double that for OcuMel, the charity that helps sufferers of ocular melanoma."
Andrew Sutton, director of Specsavers in Ebbw Vale, where the abnormality was initially picked up said: "Byron originally visited us for a routine eye test back in 2012 and, as part of every test, we take a picture of the back of the eye using digital retinal photography.
"The back of the eye is the only place where microcirculation of the blood can be observed directly and it helps us to detect and manage conditions such as diabetic changes, the onset of Glaucoma and macular degeneration.
"In Byron's case, the optometrist spotted a slight discolouration and a small object at the back of the eye and immediately referred him to a specialist.
"Fortunately, we were able to catch the tumour (known as Choroidal Melanoma as it is found in the choroidal layer of the eye) early enough for treatment to take place. In most cases there are no immediate symptoms, and tumours will continue to grow un-noticed. If Byron hadn't come to us for an eye test, the chances are he would not have discovered the tumour until it was further advanced and much harder to treat.
"The eyes are the window to the body and regular eye tests can help to determine your general well-being, as well as provide early diagnosis of serious conditions. In fact, around 50% of blindness in the UK could be eliminated if more people had regular eye tests.
"Most people tend to think of an eye test as a vision test, which of course is very important. However, there is much more to an eye examination that the health of the eyes which most people are unaware of. We recommend that everyone should have their eyes tested every two years or every 12 months if you are over 40.
"In Wales we are ahead of the rest of the UK with the introduction of the New Welsh Eyecare Initiative and Specsavers are major supporters. Many of our opticians receive further training and accreditation to do further specialist test procedures that can detect early conditions. This early detection allows our optical stores to refer patients to eye care specialist directly and quickly under the NHS system.
"We are continuing to support Byron throughout his treatment and think that it is admirable that he is using his experience to raise awareness of the condition and the importance of regular eye tests."
Hayward is currently a WRU National Academy skills coach and Wales Under 20 head coach. WRU Group Chief Executive Roger Lewis said, "Byron has the full support of the Welsh Rugby Union, both for a full and rapid recovery from his illness and for the challenges he has embarked on. It is admirable that he is using a difficult time to raise awareness of the risk of eye cancer and the importance of regular eye tests."