How should I train for a century while still commuting by bike to work?
Take a "short cut".  I commonly take a "short cut" on my 10-mile commute, riding up to a neighboring community and then down a bike trail, adding another 17 miles.

Added: The advantage to this is that you're already dressed and on your bike, so the time required will be ONLY the additional time you actually spend on your bike.  Get going 90 minutes earlier in the morning or get home 90 minutes later in the PM -- no extra changing, no extra showers, no extra hauling the bike around.