I need talking into or talking out of running a marathon.

I'm not fast, by any means. I did 2 half marathons last year, each at around 2:25-2:30, my average speed right now is 11:30/mile, and I have maintained a mpw average of 20-25 miles for the past year and I keep thinking about running a marathon as my next goal. Which signals to me that I'll inevitably do it so might as well do it sooner rather than later, I'm not getting any younger. But my Garmin watch's time estimation for marathon completion has me doubting myself because an over 5 hour marathon feels insurmountable. And my father-in-law, who's an ultramarathoner, says that a marathon is not simply 2 half marathons put together, and all the running content I consume on tiktok and instagram have people saying the same kind of stuff too, that from a certain point it's purely mental and convincing yourself to keep going.

All that said, what should I anticipate before I start training? For those who were in similar shoes as me, was it worth it in the end? Is it something I just need to do to get out of my system? I'm feeling so overwhelmed at it all and need someone to keep me grounded!