Aerobic Fitness Matters, and Running Is One Way to Build It
Aerobic capacity is a meaningful marker of cardiovascular health and functional fitness. VO2 max, in particular, is tracked by many exercise scientists as a useful indicator of long-term health alongside other measures. The research specifies that the cardiovascular system needs to be trained regularly. The modality that produces that training stimulus varies. Running is one option. Cycling is another. For some adults, running suits them well; for others, cycling or another modality fits their schedule, preferences, or joint tolerance better.
One modality difference is worth knowing if you are also lifting: a meta-analysis of concurrent training studies found that endurance work done by running, but not cycling, produced measurable interference with strength and hypertrophy gains, plausibly because running’s repeated impact loading creates more muscle damage than cycling’s concentric-dominant pedal stroke (Wilson et al., 2012, Journal of Strength and Conditioning Research). That is a reason to lean toward cycling if you are prioritizing strength and want to minimize any drag on it, not a reason running is off the table — the effect is a matter of degree, not a hard conflict.
The goal is to find an aerobic approach that is sustainable over months and years, because consistency is what moves the needle on aerobic fitness, not any single heroic workout.
For Time-Limited Adults, Intervals Beat Slow Steady Work
“Zone 2” — working at an intensity where you can hold a conversation but would not want to sing, roughly 60 to 70 percent of maximum heart rate — has become a popular default recommendation for aerobic training. But a 2025 narrative review of the zone 2 literature concluded that the evidence does not support low-intensity work as the optimal way to build mitochondrial or cardiorespiratory fitness, and that higher intensities reliably produce those adaptations too, in less time (Storoschuk et al., 2025, Sports Medicine). For someone with a genuinely limited training budget, that matters: spending your available minutes on higher-intensity intervals tends to be the more time-efficient choice, not slow steady-state work.
The practical version of this for busy adults is to treat a weekly minimum of vigorous-intensity work as the priority, not a nice-to-have layered on top of easier sessions. National physical activity guidelines put that minimum at roughly 75 minutes of vigorous activity per week (or an equivalent mix with moderate activity), and structured intervals — short, repeated efforts hard enough that conversation is not comfortable, with recovery between them — are an efficient way to hit it in a couple of sessions rather than spreading it across many easy hours.
Circuits Raise Your Heart Rate. That Is Not the Same as Aerobic Training
If three separate cardio sessions per week feel unrealistic, hybrid strength-cardio circuits are worth understanding on their own terms. The basic principle is simple: when you move between resistance exercises with short rest intervals (say, 30 to 60 seconds), your heart rate stays elevated for the duration of the circuit. That is a real metabolic effect, and it is not nothing.
But an elevated heart rate during a circuit is not the same stimulus as dedicated aerobic training. Circuits are built around load and movement, not around the sustained, repeatable effort that trains the cardiovascular and mitochondrial adaptations aerobic work targets. Treat a well-designed circuit as a useful supplement to a training week, not a substitute for the dedicated conditioning sessions or the outside aerobic work described above.
Conditioning Within a Structured Program
Most aerobic fitness advice is written for a hypothetical person with a fully equipped gym, unlimited time, and a body that responds predictably to standard protocols. Most adults who take training seriously are not that person. They have a set of dumbbells and a pull-up bar, or a stationary bike. They have weeks where they train consistently and weeks where they do not, and they need a program that has a clear structure rather than one they have to rebuild from scratch each time.
Margin’s six-week program includes two conditioning sessions per week. One is bike-or-run intervals — you pick whichever fits that particular session, not a modality you set once and keep for the cycle. The other is an athleticism and mobility session with a dedicated mobility block. Alongside both, the program expects some additional aerobic work of your own choosing each week. None of this replaces the strength sessions; it runs alongside three of them as part of the same weekly structure.
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