Chinese reproduction practices.
I wondered about that, too, as I first read it in a "Ripley's Believe It or Not" book some 53 years ago. I assumed you had 'em all lined up, winding around Peiping and back down to Shanghai, and so on. However, there is another way.
The first batch across the line could get together the day before, and then march while the next days marchers are assembling.
The next day those folks could walk across the line which the next days bunch is assembling.
They could lead their normal lives for everything except the day they marched and the previous day.
I suspect they'll be grateful for this scenario, and it will cause much less disruption than the "eternal line" model. To say nothing of the number of portable toilets that would be necessary.
Arf