"Days like this are for you (his supporters) when it's really hard to get the miles in," he said via video on his Facebook page.
"First day in California, we're going to go for a big day," he said.
"It is extremely hot, it's one mile at a time at the moment ... plenty of ice in the hat just to keep me relatively cool and, when I get back, it's in the car for a bit of air-conditioned coolness."
The endurance runner has only been in the sport for a decade but the American run has been a long-held dream. Newburn made an appearance on the local Fox News channel as he passed through Arizona earlier in the week.