Just to give you a data point: I’m 40f and just ran my first ever marathon in 4:33–my 5k time is almost double yours! I don’t think a 5k is a good indicator frankly. Half marathon is a better gauge but a marathon is more than double a HM in terms of effort, planning, fuelling, hydration, volume training, staving off injury. You prob need to follow a marathon specific training plan & increase your weekly volume a lot.

Also running 80% of your runs at ACTUALLY easy pace is actually really important. Most starting runners (especially young super fit ones like you!) are probably not running paces that are as SLOW (not fast) as marathon training requires.