Pregnancy is like marathon training. It's EXACTLY like marathon training. You have this thing that's going to happen in the future, and you have months to prepare for it. There are books to read, foods to eat, exercises to do (or not, whatevs)... Instead of increasing mileage, there's increasing pounds on top of your body. It just gets heavier, and harder to move around. But the harder it gets, the closer you know you are to the finish line.
Also- there's the one big push at the end. The hardest day is the last day. And it's coming. And you know it's coming. And you're kinda scared, and pretty excited, and you're not quite sure if you'll be able to handle it, but you really can't turn back now.
The main difference???? Scheduling. If I told you someone was going to come to your house with a bull-horn and announce the start of your marathon while you were sleeping, or working, or shopping, or driving- and you wouldn't know when... just a vague idea of sometime in the next 4 weeks.... and then ready... set... GO! Marathon! 26.2.
The second difference??? The actual mileage. Maybe labor will last 5 hours (yay!! Sawyer was so easy!) or 15 hours, or... sigh... possibly closer to 20-24 hours. You won't know until it's over how long it actually will be.
Another similarity... pulling the plug. What if something weird happens and they have to do a C-Section? Breech baby? Weird infection? You'll get a DNF (did not finish) for the laboring, but you'll still get your medal for trying.
More similarities: heat an humidity are enemies of both. You need new, life-event-specific clothes that won't really be applicable at any other time. Hydration is key, as is going with the flow: preparation will only get you so far during the actual performance.
Final similarity: you want pictures of the whole thing, but the pictures will come out kinda wonky due to sweating.