You definitely have enough time to train up to 80 miles. The tough part for you is that since you're only 18 years old you don't have much of a fitness base to draw on. So you need to build that base. A couple of focused training rides per week, one with hills where you push a bit on the hills and one longer ride. If you haven't ridden at all in the last 2 years, start at 45 min and add 5 to 10 min each week, until you get up to about 90 min then you can add 15 min per week. Those long rides should be an easy effort, you should be able to talk and laugh with your riding buddy. Then try to get out on your bike another 2 to 3 days during the week even if it's just for very short, easy rides. These aren't supposed to be training rides... just for fun. A good way to do this is to use your bike for commuting if you can. Ride your bike to work or your friends house... you get the idea.
Have fun!! Cycling is great and what a fantastic goal you have.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS