To the opening post, I began cycling just before new years and am also 18 years old.
100 miles a weekend is roughly what I do aswell, and the distance is fine. I try to get out on my bike during the week but the january exams have other ideas, so it wont be long until I am out almost every day.
I aim for a 20mph average no matter what ride I do, and if I see my speedo drop below 20 a piece of me dies inside. Luckily base fitness before I began was pretty decent, as I have been into long distance running for quite some time - though various injuries have held me back (joys of shin splints..).
Best way to train is with others, and when they say "are you sure you can keep up?" just go for it...even if this does mean you are in a pool of vomit, halfway up that bastard hill 40 miles into the ride.
Wish you best of luck. It would be nice to see how you are doing every now and again as you are in a similar situation to me