Good thread (I read the whole thing, having just finished reading a cycling book and enjoying having some more cycling stuff to read). I learned here that my hybrid is really a "fitness" bike. It's funny that without knowing it I picked just the bike I would have wanted. I even got the right size, not that the store gave me a choice anyway. I'm average in height and weight for an American male so I guess that helps a lot when buying a bike off the rack.
One thing I'm kind of put off by in the thread is the assumption that because we're newbies and in our 50s that we're out of shape and seeking to make bicycling easy. It's true I'm new to biking but I've kept myself in decent shape doing other things and I'm looking to bicycling to help me get in better shape. I have little interest in using lower gears or getting a faster bike just to make my work easier. Of course, I have nothing against using lower gears for hills or sprints and for getting a new bike for longer, harder rides.
It would be great to have folks continue to add knowledge and links to this thread. I feel it's always better to have more info than not enough. For my part, I've only been riding for four months but, nonetheless, I could put a plug in for hybrids. They're a lot less scary than the superfast road bikes with skinny tires for beginners and one can also enjoy stonedust trails with impunity. You will be limited to about 15 mph on the speed front, if that's an issue, especially if you load up with a rack and supplies for the day.
I'd also like to put in a plug for a really old book I just finished reading from my library. I don't even think you could buy it anymore. It's an engaging account of the author's rides across the US, both top to bottom, and across. After reading about all his training, it really shamed me into upping my own mileage quite a bit. Like the author stated, much of one's limitations are just in one's mind (assuming you're in decent shape). The book is 'Sport Cycling,' by Michael Shermer, pub 1985. He also gives a lot of riding tips and biking explanations that are still valid today.
Happy riding! Ron