I have done a few sprint tris now and have decided to step up to a half ironman. Do i need to alter my training, besides distance? I currently run, swim and bike 3 times a week, plus tomgym workouts in a week. I figured i could just increase my distances.

More distance, obviously, but you also need to work on your nutrition intake. You can easily do a sprint wihtout taking any fluid or solids in. A half is different. Find foos that work for you and whatever combination of sports drinks that works for you over long periods of time, You are looking at somewhere between 5-8 hours of exertion for a half, so you need to have a plan in place for that length of time, and practice it prior to the event.

Long distance triathlon running is about running well on fatigued legs. You already have the gym workouts to increase strength but I would increase the percentage of cycling and running that you do on hills to help prepare yourself even more to run on tired legs.

caelric has it right that you need to practice nutrition. Get a bento box for you bike. I love honey stinger products (waffles and chews on the bike and there gels for the run). The way you train should change slightly beyond just adding distance. I would make sure that you include brick workouts (run off the bike) if you don't already. Remember that they way you race is much different. For a sprint you can pretty much go all out start to finish, but for a half or full you need to never push it too hard. A HR monitor is very inexpensive and helps to isolate which zone you are working in. Again, if you don't already have one you should consider getting one and using it.

I downloaded a HIM plan off the internet and it worked well for the training. The most difficult part for me was getting my nutrition right. I used Lara bars and GU with water to drink. I learned the most about what worked for me the last month of training before the race. I did one indicator brick a week (60-65 mile ride and a 4 mile run) the last month and found Gatorade, which had worked fine on Oly distances, gave me bad cramps on the run. I planned for 200 cal/hr, but you will need to find what will work for you.

All good advice. On your first HIM don't set a finish time just race comfortably. One key is pre race nutrition. You will need to experiment with foods that you can tolerate and will not cause cramps. During the race I like the Hammer Perpetuem and any kind of gels. I'll also take peanut butter sandwiches and a banana on the bike leg. Have fun.