(CNN) - Rolling Stones frontman Mick Jagger is back in the studio and says he's "doing well" after his heart surgery.
"I'm feeling pretty good," he told Rolling Stone. "Been rehearsing a lot lately in the last few weeks ... This morning [I did] a bit of gym. Nothing crazy. Then I go into rehearsal with the rest of the band."
Jagger underwent heart valve replacement surgery in April and the Rolling Stones had to postpone their North American tour as a result.
But Jagger promised the group has a lot in the works for their fans when their tour resumes.
"We're trying to pick some ones we haven't done in recent years, stuff we haven't done before," he said. "Most of the time people don't want too much unusual. People like a little bit unusual. They don't want 100 percent unusual."
Jagger added that "Memory Motel" is one of the rarities the band rehearsed a few days ago.
Related: Mick Jagger doing well after heart valve replacement
Before his recent medical treatment, Jagger, 75, took to Twitter to apologize to their disappointed ticket holders.
"I really hate letting you down like this," he wrote. "I'm devastated for having to postpone the tour but I will be working very hard to be back on stage as soon as I can."
But his bandmate Keith Richards says they always knew Jagger would be back and roaring to go again.
"He went through it very easily. He's in great shape -- rocking," he told the magazine. "As I say, it seemed to be no problem at all really." Richards said he had no fears about Jagger's mortality. "It was just, 'Oh, Mick will be better.' We'll just wait around because there was no doubt that we'd soon pick it up. It just had to be done."
The Rolling Stones, which first formed in 1962, have been nominated for 12 Grammy Awards, winning three. Their first international number one hit was "(I Can't Get No) Satisfaction" in 1965 and became the band's first gold record.
The band hits the road again later this month, beginning with a June 21 show in Chicago.