Want to know the secret to running a marathon or competing in a long distance triathlon?

80 / 20

The 80 / 20 principle.

Spend 80% of your time training in zone 2 (below your ventilatory threshold VT).

And spend the remaining 20% above.

[Where the ventilatory threshold (VT1) refers to the point during exercise at which ventilation starts to increase at a faster rate than VO₂ (V – volume, O₂ – oxygen).]

For example, if you’re training for a specific running event, say a half marathon (21.2 km) and your current program includes approximately 10 hours of training each week, then 8 of those hours should not see you heart rate climb over into your ventilatory threshold limit which could be 150 beats per minute (depending on your individual fitness level).

The other 2 hours would be spent in zone 4 or 5 doing intervals where heart rate would fluctuate over and under 170-190 range.

Where heart rate training zones are referred to as:

Z1 (zone 1) = recovery and below VT

Z2 (zone 2) = endurance aerobic and below VT

Z3 (zone 3) = active and over aerobic just on threshold

Z4 (zone 4) = hard exercise well over threshold

Z5 (zone 5) = maximum effort well over threshold