Remember time in the saddle is ultimately dependent on how far you want to go and how long it will take you. If it only takes you 4 hours to do the 70 miles then great. Jump your training rides up to 80 or 90 to get the extra time. Oh and by the way if you can ride 80 miles you should go out and try the 112 somewhere. That is only 32 miles more then the 80. All you need is to be able to do 3/4 of the distance comfortably to be ready for the whole thing. My first year cycling I did a ride that was 60 miles long and then turned around the next week and did a century. Untill then my longest ride was a 50 miler, a few weeks before.