Distance- This is probably the most obvious one. As you approach an event you need to make sure you are capable of completing the distance. For an Ironman that of course means multiple 100+ mile bike rides in the 8 weeks leading up to the race. For a 70.3 you probably want to be comfortable with 60-70 miles. It's reassuring to know you've covered the distance in training so you can at least get to the finish line if you have a bad day. Now, when it comes to the run I definitely don't recommend running a marathon in the final 2 months before an Ironman. Many people like to run a marathon earlier in the year so that they "know" they can run one. While there is definitely some stock in that argument I will tell you that it is not necessary to run a marathon before completing an Ironman. Many, many athletes have run their first marathon in the actual Ironman and done quite well with too. The run in an Ironman is a whole different ball game then an open marathon and it's tough to compare the two. With specific training you can simulate training with the fatigue your legs will have on race day through long bike rides. For many that will be much better and more efficient than training to run a 26.2 mile race and putting a lot of pounding on their legs.
Intensity- Train at the intensity you will be at on race day (or faster). This is often the reason many people experience cramps and have nutrition problems in a race. They did the long stuff, the long rides and runs. They practiced their nutrition over and over again so now why is their gut exploding out the back end? Well, it's important to include some race pace work in many of your long rides and runs so you can practice eating and drinking at those intensities. Going fast in an Ironman is about who slows down the least, not who speeds up. Those who slow down the least are usually the ones that have not only trained to cover the distance but have also trained to hold their goal pace for that distance. Again, this doesn't mean you go run an entire marathon at your goal pace but it is important to include marathon pace work in your training and off the bike. Mile repeats with short rest off a long bike ride are a great way to practice finding your rhythm and establishing a sense of pace. A 1-2 minute rest is enough time between each rep that you can reset mentally, make sure your form is good and then go again. It's an excellent way to accumulate more time running at a quality pace.
Nature of the Course- If it's hilly, practice riding and running on hills where you maintain a steady effort level up and down. If it's flat and windy then try to find a course you can ride where you stay tucked in the aero bars for 1-3 hours at a time. Some people are naturally much better at locking it in and just holding a steady effort. I'd much prefer a course with hills so I can get out of the saddle some and move around on the bike. When I race the Galveston 70.3 I have to practice the steady pacing aspect because that course is flat and BORING.
Weather- Obviously, training in the heat isn't a problem for those of us who live in Texas. I actually recommend that people try to stay out of the heat for much of their training (impossible I know) and only target certain sessions to deal with the elements. Otherwise, it's just too much being in the heat all the time and it takes a toll on your body. Training in the rain or cold deserves a mention here. I don't' like to shy away from training when the conditions are tough because you never know what fluke will come about on race day. I've raced when it's 30 degrees outside, 53 and raining, 110 and everywhere in between. It sucks sometimes but everyone has to deal with the same elements so HOW you deal with them makes a big difference. Some of my best races were the really, really cold ones because I didn't freak out and stayed very methodical with my transitions and racing and made up a lot of time while others were making poor decisions or getting flustered.
The Intangibles- Every race has something that makes it different or unique. Maybe it's the fact that it is completely UN-unique (not sure that's a word), like the Texas 70.3 I mentioned. Some races have a double transitions or you have to mount your bike on a giant hill or there are some sharp corners and 180 degree turns. Practice those things. It shouldn't take up the bulk of your training and fitness is still the most important factor in your performance but those intangibles can add up and make a big difference. Many Ironman and 70.3's are going to multiple loop courses. Are you comfortable riding around hundreds of other athletes? Can you pass others in the aero bars and take corners at high speed? Mary Beth Ellis, recently wrote a blog about her race in Panama where she admitted to be to timid on the bike and losing time to some more aggressive girls because of a crash she'd had the year prior.
So get out their and train. Think about your upcoming races. Prepare to meet the demand of race day and execute your plan. Then have fun and enjoy the ride!
