You definitely want smooth, packed surfaces to bike on, and you want these to be reasonable straight / curvy ('differentiable' may be a sufficient definition, although it's been a while since I've looked at calc).

That said - you don't need modern car-caliber roads.

I've ridden 10's of miles on packed dirt trails (in upstate NY, near one of the canals) and it worked great. Smooth, level, no sharp corners, and waaaaay cheaper than paving.