Hold a 10lbs weight on your arm for three hours (simulates an infant trying to fall asleep in your arms). Do this for six months, three times a day.
Lift a 20lbs bag of rice off the floor to your hips once every 20 seconds for two hours (simulates a toddling-walking child wanting up). Do this for a three years, six times a day.
Get up off the floor in a hurry once every five minutes for two hours (simulates the toddler-child doing something they shouldn't). Do this, I dunno, forever.
Lift a 40lbs bag of dog food over your head every five minutes for 4 hours (simulates an older child wanting to play). Do this for 10 years, five times a day.
There is a lot of other 'cardio' like things mixed in too. But the above should give a sample of the physicality of child rearing. It's a lot of low weight, very high rep, long workout, long interval, isostatic exercise. You're mostly holding things near your center of mass for very long times, adjusting between arms. And you have to get up off the floor a lot in a big hurry.
For those who want kiddos sometime in the future, maybe about 2 years from now - this won't change, 2 years from now you'll still think 2 years from then is going to be the time. Just stop and have one. It'll turn out ok, probably - and the sooner you get one, the easier you'll handle the toil, 1% of which is described by the parent post ;)
