On the map, it doesn't show very far. But according to Lonely Planet, a bus ride takes 10 to 12 hours. So I assume the road is very poor. Is it nevertheless possible to drive it on your own with a car? Are any special preparations necessary?
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Even better, it is the Pan-American Highway so it is the road and historically the best maintained in Ecuador. It is about an 8-9 hour drive, limited stops by private car. I did it at night once and it was stressful. Ill do it of days from now on. I would take the executive (Taxi bus on the sign) bus from Quitumbe as it leaves very regularly and enjoy the views. Be sure to sit on the right.
By the way, getting south out of Quito in a private car is a bear that will take many hours for the novice. My advice is "get high left or right" straight away and take these high roads, you do not want to deal with historic Quito and southern Quito traffic through the valley.