A 75 Kg. triathlete with a threshold (think maximum sustainable power for an "all-out" hour TT) power of 300W could ride the bike segment of Ironman Lake Placid at a normalized power (think weighted average power) of 240W. At this effort, "staying legal" and possessing average aerodynamics (whatever that means), he'll likely ride about 5:30. What he should hold is much closer to 220W, though. This will add about 5 minutes to his bike time...and likely save him 25-30' on the run. The reality is that even if he were to drop down to 210W, he'd still go 5:40...and possibly save even more for the run.
It never ceases to amaze me just how huge an impact riding even a little bit too hard on the bike has on an athlete's run. The longer the race the worse the damage. An athlete capable of a 3;30 stand alone marathon, if properly trained and on a reasonable day weather-wise, that has a sound nutrition plan, shouldn't run slower than 4:00 hrs...and in reality, on a flat course (unlike Placid), closer to 3:50. So, why do so many pros, and especially age groupers run so far off their marathon PR? Answer: They ride too hard. You may say, "how hard should I ride then smartass?" Don't worry, I am going to tell you.
If you're a male pro, you should be holding somewhere up to-78% of threshold power , for a female pro, 75% and for an competitive age grouper, between 68-73% of threshold. The variation occurs for many, many reasons, but in simplest terms, if you ride for 4:40 instead of 6:00 hrs. you can hold a higher percentage and still run well. Athletes with a proper understanding of how to use a power meter use their power meters as a governor to ensure they don't ride too hard. It's so common for an athlete to tell me they felt they were going way too easily early on and just how tempted they were to go harder, but how grateful they were later that they didn't.
It's so easy early on when at your fittest and having tapered effectively to feel awesome and be tempted to push too hard, but the power meter is there to tell you "whoa now, you will pay for this later". Remember: "All you can do, is all you can do". (Coggan) There are great days, and they can be planned relatively easily, but there are no miracles. In other words, the above athlete will never (yeah, I'm sayin' never) ride 80% of threshold in Placid without imploding on the run...never. Next time you watch an IM think about all those long runs, threshold runs, all that track work that each and everyone that pinned on a number has done, and then take note of how many are walking. Most simply rode too hard...it kills me to see the disappointment.
Now, for those that don't have power meters yet, think about training and racing with one. As Danielle Sullivan says, "having one is like cheating". If getting one isn't in the cards, then perhaps I can offer this advice: Ride easily for the first 80 miles (at Placid until the second time past Upper Jay) and then ride no harder than steady. Then get off your bike a run a marathon.