I have quite a bit more free time this base season than last. However, the training plan I'm following, based on the Friel book, strongly encourages a very gradual taper towards longer-mileage rides. I'm young (19) and have never had any type of overuse injury due to excessive training load. I really feel like I could be doing daily 3hr rides, since they are low-intensity (E2), and since I have the time to do them.
Would increasing the length of my base training rides at an early point in my season have a positive effect on my training? Or would I end up paying for it later because I didn't do a proper build-up?
I think you would benefit from doing the long E2 rides. That's what I did at your age. I find I benefit most from them if I "stretch my legs" a few times during the ride. Give it a gentle sprint on a short hill, try a couple rollers without shifting, that sort of thing. Just a few though, and well separated. Don't tire yourself.
After you do about 3 of these longer rides, re-evaluate. Be sure you're not whacking yourself. Watch your morning resting heart rate.
I probably would not do them daily at first. Even if you aren't getting sore, it takes our body a while to get used to that kind of riding. If you are up to it, maybe do two back to back, then a day off or a real light rest ride then repeat. Stretch a lot afterward and eat well. See how that feels, then go from there. If you have time, I don't see why you wouldn't want to do it. Getting out on the road for long periods of time is the most appealing part of riding for most people, and also the hardest thing to do schedule wise. I am actually in the same situation right now, and I am going to try something similar. I would like to hear how it works out.