I do not recommend cycling the entire Quilotoa loop, because although it is beautiful the scenery is fairly self-similar and monotous. Better to mix hiking and biking, because by hiking you can switch between the lower altitudes and the very high ones, and more easily leave the main road and see smaller villages.
Chugchilan to Sigchos is easy, almost totally downhill or flat, with a mixture of paved and (as of May, 2016) under-construction roads which are sometimes gravel and sometimes pure mud. Staying on the main road for the Quilotoa circuit is not especially difficult, but there are a few turn-offs. When in doubt, ask a local or consult a GPS. Just before entering Sigchos proper, there's a fairly fancy hacienda called Hosteria San Jose on the left hand side of the road. (I didn't sleep here but I did stop for coffee since it was raining. With rooms costing in excess of $50, this place is pricey for the neighborhood but looked very comfortable and charming. After you pass by the hacienda, it's a short trip into the heart of Sigchos, where you can find food, hotels, and wifi. There's a large public shelter/soccer field in the center of town, so if you need to stop and consult maps etc but don't intend to stay the night, that will be useful.
Leaving Sigchos towards Toacaso ("Toacazo" on some maps) is a very difficult day considering the altitude, even though the total trip is only about 60km. It starts with a very steep decent into a river valley, and then more than 40km of steep climbing. Carry plenty of water and food for this difficult day, because there's not a lot of places to get provisions in the middle. Traffic is not so bad, but there is no shoulder for most of the trip. If looking over your shoulder while you're climbing and winded makes you nervous, it would be best to start very early (before daylight) to reduce these encounters as much as possible. There are some opportunities for wild-camping if you don't want to do this entire trip in one push, but it will have to be pretty stealthy if you stay on the main road. If you leave the main road, you may find spots more easily but may have to climb even more to get there.
Detail of Quilotoa Loop
Toacaso must have accomodation somewhere, but I didn't encounter much that was open or looked appealing, and by the time you get this far you'll be going downhill or on the flat anyway. Therefore I suggest pushing all the way to the wonderful Hacienda la Cienega (closer to E35 and marked on exactly the right spot if you do a google-maps search). This place is pricey (~$80/night) but has a very classy kitchen, and I felt I deserved it after such a difficult day.
South towards Latacunga takes you back on the pan americana and E35 and a perhaps a mixture of smaller side roads. This segment is primarily flat/downhill, with no difficult climbs but no particularly exciting views. Traffic can be very bad, but the shoulder is very wide for the duration.
South from Latacunga to Ambato is a mixture of E30 and E35 and perhaps other side roads if you're feeling lucky and trying to avoid traffic. There's a toll booth somewhere north of Laguna de Yambo, right before a steep climb, at which point I left the main road and went east on a side road that took me through small villages. After being chased by several dogs and meandering through farmland, there came a very steep winding climb with a soccer field at the top where I bought ice cream and watched the game. From the prominence I decended into the small muncipality of Cunchibamba.
Entering Ambato from the North is a tricky proposition, or maybe I just took a bad route. E35 south of Pillaro made me climb, then the traffic became very bad, and at the steepest part the shoulder of the road suitable for cycling rapidly dwindled down to nothing. No choice but to alternate between awkwardly pushing my bike while standing in a triangular drainage area or pick the heavy thing up and drop it in very soft dirt, pedal for 5 minutes, then return to risking high traffic on the road because the roadside had become too thick with vegation or had a cliff and a drop off. Ambato is a beautiful city of rolling hills and river gorges, but it can be difficult to bike. If you try to avoid a congested central road, you may climb and climb only to find out later the central route is congested exactly because it's the only way to access a bridge that you'll need to cross. I suggest staying at the Hotel Ambato even though the rooms are basic and the price is high ($50 or $60 a night) because the central location and the city view it provides are both excellent.
Ambato to Banos is mainly downhill and flat, with beautiful views out over a green valley. At the bottom of what was mainly a decent, I crossed a bridge over a river and looked up to see a hollywood-style sign with letters reading "Banos". This marks the beginning of a short but steep accent, then it's downhill again until you coast into the city center. Banos is a touristic place with zillions of discos, restaurants, and hotels, and tour companies. May I suggest staying here for a week to take advantage of a Spanish school or attempting to summit Tungurahua?