You rack the bike. You pull the shoes on. You go.
And your legs belong to someone else. They are there, you can feel them, but they take no orders. Like running through sand on polystyrene legs.
The sport has a name for it, and an explanation with nothing mystical about it.
What actually happens in your legs
For six hours you pedalled. A circular movement, concentric, no impact, a small joint range, a steady cadence around 85 to 95 rpm. Your nervous system learned that pattern and put it on autopilot.
Now you ask it for a ballistic movement, with ground contact, an eccentric contraction on every step, and a completely different hip range. The motor wiring does not switch over on demand.
Add vasoconstriction: the blood was committed to one muscle chain, it has to redistribute. Add the glycogen you already burned. Add the folded position you held for hours, which shortened your hip flexors.
Jelly legs are those four things at once.
Nobody watches this: your cadence collapses
Here is the most reliable signal, and the most ignored.
Off the bike, your run cadence drops. A runner who naturally turns over at 175 steps per minute ends up at 155 or 158 without noticing. He thinks he is running normally. He is plodding.
The problem is what that low cadence does mechanically. A slower stride means a longer flight time, a higher landing, a foot planted further in front of your centre of mass. Every step becomes a brake and a shock.
Jack Daniels filmed the distance runners at the 1984 Olympics. Exactly one athlete, across the whole field, turned over below 180 steps per minute. And they held that cadence whatever the pace: to go faster they lengthened the stride, never the rhythm.
So your instruction coming off the bike is not a pace. It is a cadence. Aim for 175 and up, even if that means ridiculous little steps for the first five minutes. The pace comes back when the legs wake up.
Fifteen to twenty minutes is enough
The diligent triathlete’s mistake: stacking a four-hour ride and a one-hour run to “simulate the race”.
What you mostly simulate is an injury. And a recovery week you never budgeted for.
Physiologically, the adaptation to the transition comes fast. Fifteen to twenty minutes of running after the bike is enough to train the switch. Past that you add mental benefit, a bit of confidence, and a lot of residual fatigue. Friel is blunt about it: short frequent bricks beat long rare ones.
And above all: you never do a brick as long as the total you are racing. An age-grouper simulating 180 km of bike plus 42 km of run in training is not doing a session, he is racing without a number, and it takes him three weeks to recover.
The only brick that really counts
If you want one reference session, the one that prepares race day without replaying it, it looks like this.
Short bike but at race intensity, then run immediately at race pace. What matters is not how long you rode, it is the state you get off the bike in.
On an Ironman build, that means one hour to one hour thirty on the bike including a 40 km block at your target power, then 40 minutes of running at your target pace. On a half, drop to 30 km and 30 minutes. The transition has to be real: shoes laid out in advance, you do not sit down, you do not stop for a coffee.
The rest of the time, at the end of a long ride, ten minutes of easy running off the bike keeps the pattern alive. Feet out of the shoes over the last few hundred metres, to rehearse the dismount.
Running is the sport that will injure you
Cycling and swimming tire you out. Running breaks you.
At equal cardiac load, the mechanical cost of running is three to five times higher: impacts, eccentric contractions, repeated shock on tendons with no suspension. Your heart adapts in weeks. Your tendons and your bones adapt in months.
This is the absolute trap for a cyclist moving to triathlon. You have an advanced engine and beginner legs. The heart writes cheques the structure cannot cover. You can run for an hour without getting out of breath, so you do it, and six weeks later you have shin splints or an Achilles tendinopathy.
The rules that protect you:
Do not raise your running volume by more than 10 % a week, and hold plateaus. Three to four weeks at the same mileage, then you go up, then you stabilise again. That ramp is stricter than the one on the bike, because impact does not forgive.
Always restart from what your legs have actually done running recently, never from what your cardio would allow. You were doing two twenty-minute runs a week? You do not start at three hours of running a week, even if you feel capable.
One day a week with zero impact. Bike, swim, or nothing.
An Ironman marathon pace is not a marathon pace
Another illusion to kill.
Your standalone marathon pace, the one you hold fresh, on rested legs, with a number on your chest and a crowd: you will not see it again after six hours on the bike. An Ironman marathon is run clearly slower, in the bottom of your endurance zone.
Your limiter on that marathon is not your cardio. It is accumulated muscle damage and fuel. Your quads have absorbed tens of thousands of contractions, your stomach has been working since morning.
Which means an Ironman marathon is decided on the bike, and that is another article.
The Cornerman verdict
Your jelly legs are not a lack of character, and they do not get cured by suffering more.
They get treated with short frequent bricks, an obsession with cadence the moment you step off the bike, and monk-grade patience on the way you build running volume.
Your engine is ready. Your tendons are not. They set the calendar, not you.