Ok, I'll speak from my experience. Yes, babyhood is hell in some ways. They don't sleep through the night for several months after birth (on average), so someone usually has to get up with the little bugger every night for a while. So the unfortunate truth is that sleep will be at a premium for several months. But after a few months they sleep all night and you're good to go....until about six in the morning.
As it shook out with us, I would get up with the kids at about 6:00, I'd make us breakfast, and we'd hang out until Mom got up. (This was her break from taking care of them during the week; on my salary we can just manage the stay-at-home mom thing).
There were several unexpected things I found hardest about parenting. One was the whole 24x7 nature of the job: even when the kids are in bed, you're still (potentially) on duty. You can be happily enjoying your favorite pre-kids activity, whatever it is, then suddenly brought back to the parental grindstone by a wail from upstairs.
Because of that 24x7 thing, for a few years it made it almost impossible to really focus on projects. You know how you set out to get something done on the computer, or whatever, but something that you have to focus on to get done? Well, knowing in the back of your head that you can be interrupted at any time gets in the way of concentrating on those projects and getting them done. That fades after a while; several years after the kids were born I was able to focus on projects just fine.
Also because I was getting up at six on the weekends in addition to during the week for work, my body clock got into that habit. So once the kids were taking care of themselves in the morning (getting their own breakfast, playing quietly so Mom & Dad could sleep), I woke at six anyway. It's taken several years to be able to sleep in again on weekends.
Oh, I should mention the elephant in the room. If you're the sort of guy who's interested in sex (and who of us isn't?), having kids is going to Ruin Your Shit. Remember, when a woman hosts a pregnancy, her body gets flooded with all sorts of hormones etc. that are intended to get her through the pregnancy, not get her into another (i.e., not geared toward the activity that causes pregnancy). And then she's flooded with hormones for the care of the results of the pregnancy. That's kinda nice at first, because it means the Titty Fairy comes, but now those are for the baby.
And mom's not interested in the activity that could get her pregnant (i.e. sex) but in dealing with the Little Pooper. So even if she assures you in advance that having kids won't interfere with your sex life, IT WILL. She doesn't mean it to; it just happens. It's not her fault--she didn't mean it to be that way, but the end result will be that your sex life may well take a tumble. YMMV, and that depends sharply with age, I think. We started late, so our marital bed is pretty sterile, though from what I hear that's not universal. So at your age it'll probably be a temporary lull. Just be aware that it's very likely to happen, especially if she assures you it won't.
Another thing to be aware of is that having children really chops up your day, because you're forced to punctuate it with things that were no big deal before but are productions now. Going out anywhere (say, to Grandma's) is a production because it's not everyone grabbing what they need and piling into the car. You have to stop and think what you and, especially, them are going to need (pak 'n' play, books, toys, sippy cups, etc.), then gather all that stuff, and round up the kids and corral them into the car. And you know how on a given weekend you'll have a particular project that will take part of Saturday or Sunday? Now that project becomes your single goal for the entire weekend, because you just can't get as much done as you used to.