Start with the missing purpose
If the goal is general aerobic activity, duration and relative effort may help describe a replacement. If the goal is running-specific endurance, a cycling session cannot reproduce every demand of running. If the goal is swimming skill, time spent on another activity cannot replace practice in the water.
The same substitute can therefore be sensible for one purpose and incomplete for another.
A record that keeps the distinction
Instead of writing “equivalent to five miles,” a hypothetical log might say:
Forty minutes of cycling at a comfortable sustained effort, replacing the planned run because indoor cycling fit the day.
That describes what happened without claiming an exact physiological exchange.
Guideline conversions have a narrower job
Public-health guidance uses moderate- and vigorous-intensity minutes to summarize aerobic activity. That is a population-health accounting convention, not a promise that every combination produces identical sport performance or musculoskeletal loading.
For a new activity, skill can also limit the session before breathing does. A technically demanding swim may not feel like a simple aerobic replacement. Choose a mode you can perform safely and interpret its workload on its own terms, with enough detail to make future comparisons useful.
