What is a Double?

A Double Day (or simply "Doubling") is when a runner completes two separate running sessions within a 24-hour period. This is common among elite athletes and high-mileage amateurs training for marathons or ultras.

Why Run Twice a Day?

1. Increased Volume: It's easier to run 10 miles in two 5-mile segments than in one single 10-mile block.
2. Metabolic Boost: You spike your metabolism and growth hormone levels twice instead of once.
3. Active Recovery: A very slow second run can actually help flush the legs after a hard morning session.
4. Specific Endurance: Teaches your body to run on tired legs without the massive stress of a single 20-mile run.

Typical Structures

• The Hard/Easy Double: A hard interval session in the morning, followed by a very easy 20-30 minute recovery jog in the evening.
• The Double Easy: Two short, easy runs to accumulate mileage during a base-building phase.

When Should You Start?

Most coaches suggest only considering doubles once your weekly mileage is consistently above 50-60 miles (80-100 km). If you are running less than that, you are usually better off just making your single runs longer.

The Risks

• Logistical Stress: Two showers, two sets of laundry, and finding time in a busy schedule.
• Inadequate Recovery: If the second run isn't easy enough, you risk overtraining.
• Injury: More frequent transitions (warm-up/cool-down) increase the chance of structural issues.

Doubles are for building a massive aerobic engine, but they require a pro-level dedication to sleep and nutrition.