How about sticking with 20 miles a day and throw in a metric century on the weekends when free time allows to give you a little of a cushion for the days you miss? One day at a time...
Speaking of which, I'm not going to be officially part of the Strava challenge since I have no way to record my rides, but I'm using the idea as further motivation to get out to ride and plan to start out ahead with my first solo century this Saturday...assuming my LBS has a set of tires in stock as I've noticed this morning my stock ones are shot. I abused the heck out of them by going off curbs and riding off road, so I'm not too surprised they only lasted ~2500 miles. I'm considering a pair of 700x28 gatorskins for flat protection and less rolling resistance on the roads but also should be safe off-road (limestone/gravel trails).