AJ Styles spent a majority of his career traveling the world and raising the value of his name before coming to WWE. His surprise appearance in the 2016 Royal Rumble as the #3 entrywas met with plenty of fan excitement. Styles picked up the WWE United States Title twice since his debut and held the WWE Championship on two occasions which he still has to this day.
Styles is currently sitting at 383 combined days as WWE Champion. Shawn Michaels held the title for 396 days. Therefore, if Styles can defeat Rusev at Extreme Rules, he will soon pass HBK's position of 13th longest-combined reign as WWE Champion.
If AJ Styles is able to top HBK's record of combined reigns, his next target would be CM Punk who is in the #12 entry with 463 days. Styles still has a long way to go before he is close to touching Bruno Sammartino's top record of 4,040 days though with Hulk Hogan coming in second with 2,185.
(Thanks to Reddit user FalconArrow123 for the tip)