IMHO - not sure why a century is an important mark - unless you are marking age.
riding time is, being able to comfortably ride 6-8 hours could be a goal.
bike fit is especially important to be comfortable- as our bodies mature even more important.
others have suggested a steel frame - like a surly pacer or cross check & slightly wider than usual road bike tires help with comfort - 28-32 mm work well.
another factor related to fit - the fit that feels ok now - probably will evolve during the first year or so of riding - might not be the most comfortable fit next year.