I ride a 2011 Trek Navigator 1 with 7-speed derailleur. Purchased in August of 2011, I've tried to ride as often as possible. My longest rides have been approximately 12 miles in length. I've equipped the bike with SKS fenders and a rear rack. I'm getting to the point where I'd like to start taking longer rides, maybe 10 or miles each way. Is this sort of riding unfeasible on a "comfort" bike?
I would argue that there is no limitation on distance the biggest limitation you will have will be on: Speed and Comfort over time.
Comfort bikes arn't made to go fast. They are designed to be ridden around in a leisurely way. Variables limiting speed include:
Comfort over time
Road bikes and to a lesser extent mountain bikes are made to be comfortable for long period of time for more dedicated riders. Just looking at the seat as a case study we find: wide seats are comfortable immediately but after sitting on it for an extended period of time:
It's totally reasonable to ride such distances (and even much longer) on a Navigator.
I have a 2003 Navigator back in Poland. It was my "return to cycling as an adult" bike. I've done many 30+ mile rides on it in rolling terrain. I don't think it would be much fun on a super hilly ride (due mostly to the bike's heft) but even that is doable. As people have mentioned, you can't be in a hurry -- the bike is not meant to go fast.
Ride regularly, increase your mileage slowly (10% per week is an oft-quoted figure) and enjoy your bike.
One of the first rides I did with my (now) wife was a 60 mile course, which she rode on a Mountain bike. It was easy to know she was with me, because the wide knobby tires made enough noise to hear from a distance. The rolling resistance had to be sky high. She wasn't happy with the ride, but she finished.
After that ride we immediately shopped for a new bike.
The point is that it's not the bike that is the limitation, it's you in combination with the bike. If it's not working out you'll know pretty quickly, and the attempt won't kill you. Try it and see.
A twenty mile ride is definitely not unfeasible based on what you describe. You're already doing 12 miles, so adding 8 more shouldn't be a problem, depending on your fitness level and the terrain. (Terrain will be a big factor. A "reasonable distance" on flat terrain may very well be "unreasonable" on very hilly terrain.)
Anecdotally speaking... I know several people who regularly ride 20 to 50 miles on comfort bikes. I also know a few who would find 10 miles challenging. The "longest reasonable" distance is a bit subjective, but based on the description of your goal, you should be fine.
And like heltonbiker mentioned, you may want to use narrower tires and possibly swap out your seat.
I would suggest experimentation. Gradually add distance and time on the bike, and your body will let you know.
A quick note... I was just in the local bike shop at lunch picking up a tube and took a look at "comfort" bikes. My impression is that if I wanted to ride a "comfort" bike for relatively long distances... I would change the seat to something more like a road bike seat. The stock seats look fine for maybe 2 or 3 hours, but beyond that, I'd really want a skinnier seat. And depending on the stock tires, I might want to change those as well.
My 2010 Trek Navigator 3.0 is a quality bike capable of being ridden as far as your rear will stand. My longest single day ride was 50 miles on this bike. The gearing on the Trek is quite Low having a similar 34 tooth cassette as some mountain bikes and most certainly lower geared than most of the road bikesI've examined.
In 2010, I put 1500 miles on it and upgraded to a Surly LHT after I convinced myself that biking was going to remain a part of my life. I am 6'2 and ~230 so I am a big guy, but the bike did not care. Trek builds three versions of the Navigator old ones are 100 - 300 and the newer ones are 1.0 - 3.0 with the 3 series having better components, ie, shifters, seats, hand grips.
The problem for long rides with this bike is also it's advantage. According to bike shop employees, most Comfort customers ride less than 10 miles at a time and the upright position is very comfortable. Road and touring bikes distribute the weight over the seat and the bars and while it took my arms awhile to stop aching from the change on the LHT, eventually you find yourself quite comfortable in that non-upright position. Must be why they build so many that are NOT upright. The simple point is that after 10 miles my hands and my rear hurt and that means stopping more frequently than you or your friends will like on a long ride. Several 'fittings' impacted my wallet but not hands or my rear.
Personally I see the Hybrids in the comfort class, but with skinny tires and better shifters. You still ride them upright though. To be fair though, my Daughter-in-Law ~45, can ride a century on her Hybrid, and I only wish I could keep up with her for 5 miles, much less a 100.
My Navigator served me very well.