Like Mr. Silver, I did RAIN the first time about 14 months after I first started riding. I also did it again a year later. It's very doable. Training, in part, depends on "how" you want to do it. To ride it comfortably/strongly, I'd first and foremost suggest riding....A LOT. I would recommend getting some longer rides under your belt, i.e., at least 100 miles. If you can comfortably do a century, RAIN isn't that much harder. It's largely a question of figuring out how to eat and drink on a longer ride. You have to know what your body will tolerate and you also have to develop a certain discipline about staying on top of your nutrition and hydration needs, especially in the heat. That takes some practice.
It's also a question of getting used to being in the saddle for a long stretch. I didn't find RAIN all that difficult to finish to be honest with you, but my "training" for it consisted of averaging about 175 miles per week. so my legs and butt were used to a lot of miles. I don't necessarily think you need to ride that much, but you do need to consistently ride a fair amount. Obviously, if you can find a training partner(s), you may find it easier to get those miles in.
Success at RAIN is also partly a question of bike fit. If you experience any discomfort on your bike, try to get it sorted out before the ride. You really need to be comfortable on your bike if you're going to finish strongly.
There are a couple of other things to consider to do RAIN. It attracts a large group, so practice your group riding skills, even if you don't plan to do the ride with a group. You also need to consider the logistics. Getting to Terre Haute and back from Richmond take some planning. I don't necessarily think you need a dedicated SAG driver, but they can prove helpful. When I've done the ride, my driver caught up with me at the lunch stop (so that I could eat my own food), at Greenfield (which is an unofficial SAG stop just as the route rejoins US 40) and at the final SAG stop.
Finally, RAIN is partly a function of the weather--which you have no control over. If you're lucky, you'll have a tailwind and/or it won't be too hot. If it is really hot or you have to deal with a headwind or crosswind, the ride will likely be a lot harder. Mr. Silver and I are lucky in that we enjoyed some decent weather the years we've done RAIN. In 2006, the weather was horrible (95 degrees plus humidity). A lot of people dropped out of the ride because they couldn't handle the heat. I've always said that I would play the ride by ear if the weather was less than decent. I would suggest the same thing. IMO, it's just not worth it.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher