Indoor Cycling for Ironman Performance
Without a doubt, the most useful Ironman training tool in my possession is a bike trainer. After all, Ironman is - first and foremost - a bike race. To be able to run well, you've got to build your bike engine strong enough to carry you 112 miles with energy to spare. Using a bike trainer is the most efficient way to do this, especially for time crunched athletes. But even professionals, who aren't stuck at a desk for 40 hours a week, have jumped onboard the trainer train. Lionel Sanders, Meredith Kessler, and Andy Potts are some notable examples.
What makes a trainer so much better? Simply put, it isolates the workout from all external factors. There are no traffic lights, stop signs, pedestrians, cars, turns, potholes, random dogs, shards of glass, rain, snow, etc. On a bike trainer there's nothing to slow you down, nothing to injure you or cause damage to your bike, and no excuses to not ride cause the weather sucks.
In most parts of the country, finding good cycling roads is challenging. Most roads are simply too dangerous. Even if a road has a nice, wide shoulder, there's nothing to protect you from getting hit by a distracted driver, which is, unfortunately, all too common. Furthermore, there are reports of cyclists being harassed or even attacked by motorists. Protect yourself - ride indoors!
Indoor cycling also drastically reduces the prep time for workouts. All you need to do is throw on your cycling shorts, grab some water, and jump on the bike! With outdoor cycling, you've got to throw on your (weather appropirate) clothes, fill your bottles, pump up tires, put on your helmet, set your GPS, grab your flat repair kit, grab your cell phone, etc. And even once you finally start pedaling, it may be a few minutes until you actually get to roads that are suitable for your workout. It's easy to see how riding outdoors can easily add 20 minutes (or longer) to every workouts, and let's face it - you've got other things you want and need to do (Stranger Things isn't going to watch itself!) Why add all that extra time onto your workouts when you simply don't need to?
However, with all that said, the number one reason to choose indoor cycling is that it will improve the quality of your workouts. On the trainer you have 100% control over both your power and cadence at all times (and a decent trainer will let you determine resistance as well.) There aren't any random uphills or coasting to deal with on the trainer. Bike trainers allow you to totally insulate the workout from any factors not relevant to the workout itself. You can make your workouts as specific or as general as you want to. And when you're preparing for an event like an Ironman, getting in QUALITY workouts is the most important thing to do. Doing 2-3 short, quality sessions in per week is going to do a lot more for you on race day than spending hours outdoors doing imprecise workouts while dodging traffic. Those rare outdoor rides are best reserved for long, race-specific workouts during the build and peak phases of training.
When I was training for Ironman Canada I did - at most - 1 outdoor ride per week. Last summer, when I was training for Ironman 70.3 Coeur d'Alene, I rode outside twice (total!). All of my other rides were indoors. I typically do 1-2 FTP interval workouts per week, and then a longer ride on the weekend. This method of training has yielded me PRs at each distance, and I continue to see my performance improve for this racing season.
It's worth briefly mentioning the things you'll need if you want to move your cycling training indoors. At a minimum, you'll need a trainer of some sort. There are tons of options, from the very basic wind trainers to the more advanced setups with built-in power meters, and there are also rollers, which help to better simulate road-riding. I personally use a CycleOps Fluid Trainer, which increases resistance based upon how hard you pedal, just like on the road. I find that a fluid trainer is a good compromise between the cheap, loud wind trainers and the more expensive smart trainers. But there are valid arguments to be made for trainers all along the price spectrum.
CycleOps Fluid Trainer
Kinetic Rock and Roll Smart Trainer