Well, by the time he realized he didn't want to be a hero he was already strong enough to earn an easy living with low to mid level adventurer requests easily...he starts going on DEADLY MISSIONS? Why not opening a business or something NORMAL.
MC prob didn't think things through or is a tradition that the hero can become king at least through marriage. It's also not SUPER unheard of for a hero to marry into royaltyhow did he think he could become king while being an adventurer?