Sure there are other factors -- the people that you're with, the weather, and so on -- but if you are riding for any length of time on a crappy road then it is very likely that your ride will be crappy.
Of course, what makes a crappy road? Whether it's hilly or flat can be good or bad considering what you want ... or even where you are on your ride. Bad pavement may not be comfortable, but sometimes it's an indication that a road will have little or no car traffic. And having something pretty to look at is a requirement, particularly on extremely long rides like a brevet.
"What about traffic?" you ask. Well, that's a tricky issue.
While it would be nice to bike on roads with no cars, those things are usually called "multi-use trails." They can be fun, but are usually short, don't go anywhere, and are full of people walking their dogs. The Georgia randonneurs used to have a 600K that began and finished on a 30-mile section of a multi-use rail-to-trail, and while it was fun at the 6 am Saturday start it often got a little scary at the end on a Sunday afternoon.
"Okay, it doesn't have to be a multi-use trail. How about a bike lane or wide shoulder." Yeah, I've done that. There was a road in south Florida that run straight north and south with a bike lane for 20 miles, and on the Rocky Mountain 1200K we rode a couple hundred miles on the shoulder of the Trans-Canada Highway. For both of these, during much of the day there were two lanes of cars and trucks zooming past at 55 mph or more. There was also enough broken glass and bits of tires and dead animals in the bike lane or on the shoulder to keep things ... well, exciting is probably the best way to put it.
Bottom line is this: Long roads with bike lanes have them because the road is busy, and they don't want bikes in the road slowing up traffic. Roads with shoulders have them so that cars can pull into them when they break down, or so that snowplows can push ice and other junk over there. Nobody puts extra pavement on the edge of a road for nothing.
So, if you can't judge the quality of the road by the infrastructure of the road itself, how do you judge it? Well, in my opinion, it comes down to what kind of driver you get on the road, and what their typical behavior is regarding cyclists. I call this the relative number of Assholes to Saint drivers on that particular stretch of road, or "the A/S Ratio."
To determine the A/S Ratio for a road, you merely score each car or truck that goes by you on a 10-point grade. Did they give you at least three feet? Was the road ahead clear when they passed? Did they slow down enough to make sure that no cars were coming? Cars that do this get a six or seven. If they do all of this and give you a friendly wave, they get an eight. If you're fixing a flat and they stop to offer aid, they get a nine. If you're riding in the rain and they offer to carry you and your bike home, never asking which way it is or how far, and not caring that it's going to ruin the upholstery of their back seat, they get a 10. Maybe even 11.
And then there's the Assholes. If the car or truck doesn't move over when it passes or it zooms by while there's an RV coming from the other direction, they get a three or four. If they lay on the horn while doing this, they get a two.
Extremes on the scale are rare, particularly for Saints. And this makes sense, if you think about it. An Asshole is going to be an Asshole no matter what. The driver that throws something at you (score him a one) or runs you off the road or hits you (zero) is just going to do this because he or she is an Asshole. They aren't reacting to anything, but are merely being true to their own base natures.
But the Saintly driver will usually only do Saintly things when the situation calls for it. To put it another way: In a Utopian world, Mother Theresa would just have been another nice old lady.
This is why an A/S Ratio of five or six is actually a pretty good score, since that one Asshole who yelled at you to "get off the road" needs to be offset by a couple of cars passing nicely and giving you a wave (with all of their fingers).
A popular cycling road around here is Leiper's Creek Road, which only has an A/S Ratio of 4.8. Carter's Creek Road is another favorite, in spite of a score of 4.6. Hwy 100 used to be a 4.7, but since they rumbled the shoulder and built more houses out that way it's fallen to a 4.0.
On New Year's Day, I went out for a long solo ride about noon. It was a quiet day, so I went down to the Natchez Trace, getting on at Pinewood Road and going down to Hwy 7. This is about 12-13 miles, and I was passed by 30 cars, all of whom went over to give me three feet.
There were plenty of other cyclists out, too. Sometimes, this can be bad for the A/S Ratio of a road, since a car may have trouble passing a big group of riders a mile back, and then take it out on you. But every car on New Year's Day on the Trace seemed to be just out there to enjoy the drive and take it easy. For that day, the A/S Ratio of the Trace was a 7.6.
Now, I may be biased because I live less than five miles from the Trace, but over literally thousands of miles on it I still give it an A/S Ratio of 6.2. I've only personally experienced one Total Asshole -- a fellow up near the northern end who emptied the contents of his tobacco spit cup at a group of 10 riders. But this one Asshole has been easily offset by numerous Saints, since almost every time I get a flat out there (which doesn't happen much because you aren't stuck on some crap-filled shoulder) a car has stopped to ask if I needed help.
Of course, there are some parts that don't usually score as well. I mentioned the top of the Trace, but the sections where it passes through the middle of Tupelo and Jackson are often worse. On the 20 or so miles that are close to these cities the cars often treat the Trace as a major thoroughfare, whereas on the other 424 miles most drivers are out enjoying what the Trace really is: A pleasant piece of pavement passing through a scenic, quiet national park.
And that's probably what makes the Natchez Trace the gold standard for cycling road quality. There are limited amenities and the terrain can get boring, but those are both by-products and contributing factors to the limited traffic out there. The pavement is smooth, there are no horrible hills, navigation is easy, and the A/S Ratio is as good as any road that I have ever been on. If you're looking for a great place to ride a bike -- either long or short -- this is it.