Since McCullum took over following the departure of Chris Silverwood, England have won six of their seven Tests this summer, including a 3-0 series win over New Zealand.
“I thought English cricket had a lot of talent, but it has a lot more than I thought,” McCullum told the BBC after England beat South Africa by nine wickets on Monday to secure a 2-1 series win.
“I knew these guys were good, but they’re way better than I thought.
“It’s a very privileged position to go and help these guys perform. It’s been an absolute thrill.
“There are still big challenges over the next two years, but so far the summer has been quite successful.”
Captain of England Ben Stokes said he was excited for the Ashes series against Australia next year, especially after suffering a 4-0 defeat Down Under in the last edition.
“When you know you have an Ashes coming very soon, it’s hard not to watch it, especially with how the last Ashes trip went,” Stokes said.
“…I’m excited for the Ashes next summer, especially with the way we’re playing.”