It's not such a great distance to many, while it might not be common among your peers there are hundreds (thousands?) of people all over the country Audaxing most weekends. A 200k ride is 'normal', 300s and 400s are hard work but still not unsual, 600k+ generally reserved for the committed though ;-)
I sneak in the odd 200-300k ride whenever I have the the time, did a pair of 200s the week before last as well as normal commuting and mid-week rides, not meaning to blow my own trumpet as I still remember when I managed my first 100k ride and felt like I was going to die, but it is really all about what you're used to.
With regard to the speeds, well it's very route dependant.... on a hilly (they all are round here) 200k with ~2500-3000m climbing my moving average is normally in the 14.5-15.5mph region depending on the exact profile and weather/fitness. Give me a flatter ride though and I'll solo it at 16+mph, get some strong clubmates together and (depending on the mates) 18+mph isn't out of the question over that distance.
Fixed/SS doesn't necessarily end up slower either, I mean it can do on some routes, but again it's all about the elevation profile (and which way the wind is blowing!). My fastest rides have been geared, and so have my slowest. When you've only got the one gear you pretty much have to ride at a fair output, so you invariably put in decent speed if your gear is well matched to you and the route.
I think the bell curve distribution of fitness levels is skewed compared to in the past, but I don't think the peaks are any higher or lower if you know what I mean. There are fit and active people now as well as inactive, but the baseline activity levels as a population have probably decreased to the point that some efforts that were once perceived as normal now appear abnormal, if you get what I mean?
