I'm also a DC biker. I've spent the last 10 years or so riding various Cannondale hybrids through a loop from my Capitol Hill neighborhood to Dupont, to Bethesda, to Georgetown, and back. I just bought a Trek Madone 5.2 and took it for my first ride last night at Haines Point. Speedy bike, but I'm so used to straight-across handlebars that my hands got sore after a couple laps.
Am also planning/hoping to do a century. I've never done more than 35 miles in one shot, but I've done 20-30 mile loops several times a week for many years, so I think I've got a base from which to work. I'm not necessarily a natural athlete, however, so things tend to come a little harder for me. Wah wah wah.