This is at least part of a route recommended by the friendly people at Freedom Bike Rental (website).
The Via Sigchos road leaves the Carretera Panamericana/35 about 33 km south of Machachi. Go west and weave your way along a serpentine road until the town of Sigchos. Here you can take a dirt spur into the Reserva Ecológica Los Ilinizas. The road doesn’t currently appear on Google Maps but it’s there on the satellite imagery. Another option from Sigchos is to take a really windy, unpaved mountain road to Llu Llu Llama Mountain Lodge. I believe this road can be taken further south to eventually return to the Via Sigchos at Laguna de Quilotoa but, again, Google Maps doesn’t show this connection. Maybe ask at the lodge.
Quilotoa is an incredibly scenic crater lake and you can walk – possibly ride – down to the shore at Playa de la Laguna de Quilotoa. Definitely worth a visit and maybe a good place to spend the night.
About 8 km south of the Lake is the lookout point over the Toachi River Canyon. Here the road finally straightens out and the last few kilometers to Zumbahua on Carretera 30 are straighter and faster.
Check in with the people at Freedom Bike Rental for other on- and off-road exploration ideas.