Probably the biggest challenges for a longer trip are on the financial side - funding it, of course, but also making arrangements to take care of any bills or obligations at home. This is easy to resolve by arranging automatic payments, of course, provided the money's available when needed! It's a good idea to pay off as many of your bills as you can before you go, so you don't have to worry about them while you're on the road, and you don't have a mountain of debt when you get home.
Packing can be an issue, depending on where you plan to go - you'll need different clothes as the seasons change. I traveled from April through August, beginning in northern Europe, so I definitely needed sweaters and warm gear at the beginning, but I mailed them home when the weather turned better. Also in the package, I mailed everything I'd accumulated but didn't actively need (souvenirs, maps, notebooks, etc.) to cut down on the weight of my pack.
Pace is important too. If you plan on visiting a new place every day or two you'll be exhausted half way through your trip. You'll have the luxury of being able to stay put for 4 or 5 days, or much longer if you fall in love with someplace, and that's key to enjoying a longer trip. It's also nice to plan in some "vacation" time, days or weeks where you just plan on visiting some nice beach or chilling in the mountains somewhere, because sometimes you get burnt out trying to see every sight and check out every attraction.
I wouldn't worry too much about loneliness, usually the hostel crowd takes care of the company factor, but you might consider asking a friend to meet up with you for a week or two somewhere, to recharge your battery if you're afraid of homesickness.
Make sure you have travel insurance in case you get sick - it doesn't happen often, but the longer you're away the greater chance there is for a cold or some weird bug to find you...just consider how often you get sick at home in a six month period, particularly if you're not eating/sleeping as well as you normally do...
Insurance is cheap and it beats paying hospital bills or a flight home out of pocket.
Ask someone to give you a rechargeable phone card so you can call home. Parents are generally good for this one, as they usually like to know if you're still alive.