There's a performance floor below roughly 25-30km for the longest Ironman training run, but no evidence that running further than that helps — and the size of a single session's jump, not the longest run itself, is what's linked to injury
Interpretation
Fokkema et al.'s prospective cohort of 997 recreational runners found a longest run under 25km was associated with a slower marathon (+13.44 minutes vs the 30-35km reference), but runs beyond 35km showed no further performance benefit and no higher injury odds. Frandsen et al.'s much larger cohort (5,205 runners, 588,071 sessions) found injury risk tracked the size of a single session's jump relative to the runner's own recent longest run (HR up to 2.28 for more than doubling it) — not week-to-week volume change. Knechtle et al.'s review of Ironman-specific predictors found training volume and speed predict finish time, but doesn't identify longest run specifically as a predictor — directly relevant since none of the volume-predictor evidence is from Ironman training itself.
Practical implication
Set a longest-run floor around 25-30km for Ironman marathon preparation rather than chasing ever-longer runs, and manage injury risk by capping how much any single session can exceed the athlete's recent longest run — not by a fixed weekly-increase percentage. All three studies are marathon or general-running cohorts; none are Ironman-specific.
Sources
3 sources, independently verified.
A longest training run under 25km was associated with a slower marathon (+13.44 min vs the 30-35km reference group), but runs beyond 35km showed no further performance benefit (-4.86min, not significant) and were not associated with higher injury odds (OR 1.01). 55.1% of marathon runners in the cohort had a new running-related injury, but no training characteristic — including longest run — was associated with that injury rate. Evidence for a performance floor, not a ceiling, below which longer runs help.
A single session more than 10% longer than the runner's own longest run in the previous 30 days carried a significantly higher injury hazard ratio — 1.64 for a 10-30% jump, 1.52 for 30-100%, and 2.28 for more than double — while week-to-week volume change showed no such relationship. This is the TriForward-referenced study behind 'forget the 10% weekly rule': what matters is the size of a single session jump relative to recent long-run history, not a smooth weekly progression number.
A 35-year narrative review of Ironman performance predictors: training volume predicted finish time more strongly than training speed, and running speed in training was linked to marathon split in women and overall race time in men. Notably, the review does not identify longest training run specifically as a performance predictor — it's volume and speed that carry the predictive weight in the literature, not the single longest session.