"It's stunning, unbelievable. I knew New Zealand was a pretty, untouched sort of place but some of the sights we've seen in the last few days were incredible," said Williams.
The pack started last Monday and cycled a 400km loop of the North Island, starting in Auckland and finishing in Rotorua, and intentionally planned it to coincide with the Lions tour.
"It's not very flat, it's quite hilly, but it was good.
"We had people from all walks of life taking part, aged from 20 to 70 years old," said Williams.
While he came away from the trip with fewer bruises than he would after a rugby game, he said physically it was just as tough.
"It was hard but well worth it.
"One of the highlights was seeing the look on everyone's faces when they had actually completed it."
He took up cycling as a way of keeping fit after retiring from professional rugby, however he and Rush still play club-level rugby in Wales.
Williams plays for Swansea and Rush plays for their rivals Cardiff.
"I'm used to him running at me on the field," said Williams.
"There's always a lot of banter, we've always got on really well.
"He's a great tour guide, he's very knowledgeable.
"You think of forwards as dull and quite boring, not really knowing much apart from being big and strong, but he was great.
"Everywhere we've gone he seems to know everyone."
Williams worked with Velindre when he was playing professional rugby and has maintained the relationship since retiring.
"They are the charity I worked with when I was playing for Wales and having worked with them and seen the amazing work they do I just carried on being involved.
"Any charity is a good charity, but the people that do this, the work they do and the people they help along the way, I just think it's incredible."
The group of cyclists have raised more than $117,000 for the centre so far.
