The brick session most amateur Ironman athletes get wrong.

Most amateur Ironman and 70.3 athletes run the brick session backward. Long ride at easy pace, then a long, hard run off the bike. The session feels like a fight. The adaptation is small. Here is what a real brick is for, and how to structure it across a build cycle.

You finished the long ride at endurance pace. Felt great. You racked the bike, laced the shoes, and went out hard. Two kilometres later your legs cramped, your form fell apart, and the run felt like death. You called it a hard brick. You posted the photo. You did the wrong session.

This is the most common mistake in amateur Ironman and 70.3 brick training, and it is preventable.

What a brick really is (and what most amateurs think it is)

The word brick is contested in triathlon coaching. Some say it stands for bike-run-ick because the run feels like a brick is tied to your legs. Most coaches we read agree the modern, useful definition is something simpler: a brick is a training session where you do a run immediately off the bike, with a specific physiological purpose tied to race performance.

Most amateurs treat the brick as a test. Can I finish the run off the bike? Race day will answer that question. Training is for adaptation, not for testing.

A brick session in a serious training week is for one of three things: teaching the body the neural transition from cycling to running (different muscle recruitment, different stride mechanics, different cardiac demand); specifically adapting the running economy under accumulated fatigue; or practicing race-day fueling, pacing, and gear in the order they happen.

It is not for proving anything. It is not for crushing yourself. And it is almost never about volume.

The mistake: too much volume, wrong intensity

The typical amateur brick week looks like this: long ride Saturday (4 hours, endurance pace), 30 to 45 minute run off the bike, lots of pride, lots of fatigue.

Here is what is wrong with that, in plain language. The long ride is supposed to be the anchor of your aerobic base. At endurance pace it is doing its job: building capillary density, mitochondrial volume, fat oxidation. Good. Then the run comes off it. Forty-five minutes is far too long to add to a 4-hour ride if the goal is transition specificity. After about 15 minutes, the neural adaptation signal is gone. After that you are mostly adding fatigue that costs you Sunday's session.

The opposite mistake is the short brick at the wrong intensity: 10 minute run at threshold off the bike. Now you are adding intensity to legs that are already glycogen-low and fatigued. The dose is wrong. The fatigue is real. The adaptation is small.

The fix is not to do more or less. The fix is to match the brick to the week.

The intensity rule: race-pace bike, easy run (or the other way around, on purpose)

Friel covers this in The Triathlete's Training Bible (5th ed., 2024) and the consensus across endurance coaching points the same way: the brick has two productive shapes, and they are nearly opposite.

Shape A: race-pace bike, easy run. This is the most common one in a serious 70.3 or Ironman build. The ride is at goal race intensity (or slightly below for the long ride), and the run is short (10 to 20 minutes) at conversational pace. The point is to teach the legs the handoff. The cardio system stays high. The neuro-muscular system practices the discipline switch. You are not racing the run. You are practicing the change.

Shape B: easy bike, race-pace run. This is rarer and used closer to peak weeks. Easy 60 to 90 minutes on the bike, then a 30 to 50 minute run at goal race pace. The point here is to teach the run under accumulated aerobic fatigue without massive glycogen depletion. You arrive at the run with tired legs but a full tank. The pace stays honest.

What almost no productive brick looks like: long endurance ride, then long hard run. That is two separate sessions glued together, and the body responds with two separate adaptation signals fighting each other.

The transition window: 90 seconds vs 10 minutes

There is real physiology to the brick transition window. The first 90 seconds off the bike are when the cardio system shifts from cyclic (no eccentric loading, supported body weight) to running (heavy eccentric loading, full body weight, different cardiac venous return). Your HR can spike 10 to 15 bpm in those seconds without any change in effort.

After about 10 minutes, the body has fully adapted to the run. The cardio output settles. The legs find their stride. You are basically running fresh from that point on.

This is why a very short run off the bike (10 to 15 minutes) is so valuable in a build cycle. You are training the body to handle the transition itself, which is the part race day will test most.

It is also why the standard amateur brick (45 plus minute run off the bike, hard) is mostly fatigue: after the first 10 minutes you are no longer training the transition. You are training a tired-legs long run, which has its own use, but is not what most athletes think it is.

How to structure brick sessions across a build cycle

A 16-week 70.3 or 20-week Ironman build cycle that wants brick adaptations does roughly this:

Weeks 1 to 4 (base): zero formal brick sessions. Establish the aerobic base on each discipline separately. The neural transition will come quickly later. There is no need to compound fatigue early.

Weeks 5 to 10 (build): one short brick per week, Shape A. 90 minute ride at endurance pace with the last 20 minutes at race pace, then 12 minutes run at easy pace. Total brick time around 100 minutes. This is the high-leverage block.

Weeks 11 to 14 (specific): one brick per week, Shape B. 60 to 80 minute ride at easy pace, then 30 to 40 minute run at goal race pace. The pace target is hard. The legs are not destroyed.

Weeks 15 to 18 (peak and taper): one short brick, Shape A, at full race intensity for both bike (race-pace, 30 minutes) and run (race-pace, 15 minutes). This is the dress rehearsal. After taper week, no more bricks until race day.

Notice what is missing: the kitchen-sink brick. The 4 hour ride plus 45 minute run. It does not appear because it is not what bricks are for.

This does not mean you do not do long sessions. The long ride still happens (in its own session, mostly endurance pace, sometimes with race-pace blocks at the end). The long run still happens (in its own session, easy pace). But you stop trying to do both in the same session and call it training.

The brick the day before the long ride (advanced)

A pattern Friel and Sutton among others note for advanced solo athletes: a short, race-pace brick on Saturday, then the long ride Sunday at endurance pace. The Saturday session primes the neural transition. The Sunday session puts in the aerobic miles. The two sessions reinforce each other instead of competing for the same recovery window.

This is more sophisticated and only works if your weekly load can absorb it. Most amateurs do not need it. Some peri-build advanced athletes use it as the last lever before peak.

What changes inside an adaptive plan

This is what we build TriPaced around. When the plan prescribes a brick, it prescribes the shape (A or B) and the dose matched to where you are in the build cycle. The ride duration, the run duration, and the intensity targets are all specified in plain language. The plan does not say brick day, you figure it out.

When you log the session, the engine reads how it actually went and decides whether the next brick can stay in shape A or whether it needs to drop to a softer dose. After three brick weeks of clean execution, the plan can graduate you to shape B without the heroic version most athletes invent on their own.

You are not winging it. You are not pushing through the wrong session because the plan said brick. You are doing the brick that matters for the week you actually have in front of you.

Show up. Adjust. Never skip a streak.

A productive brick is short, intentional, and matched to the week. A wasted brick is long, generic, and pushed through on pride. The difference is not in how hard you trained, but in whether the session matched the adaptation the body could use.

Most amateurs treat every brick as a test. Solo Ironman finishers who arrive at race day fresh treat every brick as a practice. The race tests you. The training does not need to.

The next time the plan says brick, ask one question first: what shape, what dose, what week am I in? If you cannot answer, the session is wrong before you start it.