Since McCullum took over following the exit of Chris Silverwood, England have won six out 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’s got a lot more than I thought,” McCullum told the BBC after England defeated South Africa by nine wickets on Monday to secure a 2-1 series win.
“I knew these guys were good but they are a lot better than I thought.
“It is a very privileged position to be in to go and help these guys perform. It has been an absolute thrill.
“There are still big challenges in the next couple of years but for now it has been a pretty successful summer.”
England captain Ben Stokes said he was excited about the Ashes series against Australia next year, especially after they suffered a 4-0 defeat Down Under in the last edition.
“When you know you’ve got an Ashes coming up very soon, it’s hard not to look at it, especially with how the last Ashes trip went,” said Stokes.
“… I’m excited by the Ashes next summer, especially with the way in which we’re playing.”