These "B" races can also be used to check out the course for an upcoming "A" race, as some Ironman races also have 70.3 events on the same course. They can also be used as a solid brick effort or a race-day rehearsal (dialing in pace, nutrition, equipment, etc.) for an upcoming "A" event.
Athletes need to look at the cost-benefit of these races. On the cost side of a longer-distance "B" event, an athlete may require more rest time leading into and recovery time after the race than would be typical of a break-through workout. So, by inserting a "B" race into his seasonal schedule, an athlete could be losing important training time needed to prepare for an upcoming "A" event.
On the benefit side, a short-course athlete could schedule a couple of "B" sprint events or one Olympic-distance race leading into the "A" Olympic event without sacrificing training time.
Finally, "C" races are "train-through" events that we look at as a hard workout, where we may focus on a specific aspect of the race, such as holding a certain pace for the run portion or maintaining a particular wattage for the bike portion of the event.
These efforts should be considered part of the break-through workouts for the week, for which 1 to 3 active recovery days may need to follow.
For long-distance athletes, I like to use single-sport events (open water swim, metric or century ride, half marathon) as "C" events in the base periods leading into the "A" race. For short course athletes, "C" events could be single-sport efforts as well (master's swim meet or open swim, cycling time trial, or 5K and 10K running events) or short-course multi-sport events like duathlons.
So, an athlete's first step in deciding the appropriate number of races for an upcoming season is to choose the "A" events and then build the annual training plan (ATP) backwards from each of those. Once you know where the "A" races are, you can fill in with other "B" and "C" events as they fit your goals for the season.
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