i think they say to not increase more than 10-15% or you risk injury. I know that back in 2009 I did my longest ride ever it was a metric century that ended up being 73 miles it took much longer than i expected. I think riding at 10 mph for a century could a bit much. I would look to increase your speed on some shorter rides. 20-40miles at a time until your average is quicker. I think you can do it but it would be torture to ride that long and slow. the event might be all cleaned up by the time you get back. that is a dangerous situation. just my initial thoughts