Alternations and Float Recovery: 100/90 km Repeats

The idea: ease off, keep running

In a classic interval set, you walk or jog between repetitions. In an alternation session, you run the recovery at a lively pace, so the whole session works as a continuous run with changes of gear. That recovery is called a float.

A half marathon lasts more than an hour and you never get to stop. Alternations teach you to produce race pace while you clear fatigue at a moderate pace.

Kilometer alternations: 100% / 90%

This is the most important session of a half marathon plan. You alternate 1 km at 100% of half marathon pace with 1 km at 90%.

For a 1:28:40 half marathon (4:12/km):

• At 100%: 4:12/km.
• At 90% (float): 252 s x 1.10 = 277 s, 4:37/km.

One repetition (1 km + 1 km) takes 8:49. Eight repetitions make 16 km in about 70 minutes.

Progression

1. 6 x (1 km at 100% + 1 km at 90%). 12 km. This is the first session of the specific phase.
2. 8 x (1 km + 1 km). 16 km.
3. 10 x (1 km + 1 km). 20 km.
4. Segments of 2 km at 100% with 1 km at 90%. For example, 5 x (2 km + 1 km) is 15 km. At 4:12 and 4:37, each cycle takes 13:01.

Extend first and speed up second. Keep the 100% until you complete 10 repetitions without fading, then move to 102%. In the final weeks, bring the float closer to 92-95% before you speed up the repetitions.

Long repeats with floats

Same style, longer segments. For a half marathon at 4:12:

• 7 x 2000 m at 100-102% (8:24-8:14), with 400 m recovery in 2 min. Total work: 14 km.
• 5 x 3000 m at 101% (4:09/km, 12:28 each), with 1 km at 85-87% between them (4:45-4:50). Total: 19 km.
• 3 x 5000 m at 99% (4:14/km, 21:13), with 1 km at 85%. Only for runners with more than 12 weeks of preparation at an intermediate-to-advanced level.

The float paces in these sessions run slower than in the 1 km alternations because the repetitions are longer.

Floats in threshold work

Threshold work happens near the pace you can hold for about 60 minutes. For a 40:00 10K runner, that pace sits close to 95% of 10K pace, about 4:12/km.

A 40 minute continuous run at that pace is a hard session. With floats, the same load splits into segments and you run the recovery without stopping:

3 x 3 km at 95% (4:12) with a 1 km float at 85% (4:36).

• Segments: 3 x 12:36 = 37:48.
• Floats: 2 x 4:36 = 9:12.
• Total: 11 km in 47 minutes, without a single stop.

If you are newer to this, start with 4 x 2 km and 1 km floats. Lengthen the segment to 3 km, then shorten the float to 600 m before you raise the pace.

How to run the float

• Keep it above 80%. Below that you are back to a classic pause and lose the stimulus.
• Hold the float under 90% early on. A float at 95% from week one leaves you with no strength to finish the repetitions.
• Run the repetitions at their pace. Runners often open at 103% to “bank time” and pay for it in the last repetitions.

Where it fits in the week

A half marathon week with alternations in the specific phase:

• Monday: easy run, 10 km.
• Tuesday: warm-up + 100/90 km alternations (the day’s work).
• Wednesday: easy run, 8 km with 6 strides.
• Thursday: easy run, 10 km.
• Friday: specific speed: 6 x 1000 m at 105% (3:59) with 2:30 rest.
• Saturday: easy run.
• Sunday: rest or a very easy run.

After a big alternation session, leave 2 to 3 easy days before the next demanding work. The long fast run takes another week of the cycle.

What the session trains

An alternation set trains two skills at once. The 100% kilometers build tolerance for half marathon pace. The 90% kilometers teach you to settle and breathe at a brisk pace, which is what you do in a race after a surge or a hill. Full stops teach you to rest. Floats teach you to keep moving.

Adapting the session to your level

If you run at base level, skip the 6 repetitions at first. Begin with 4 x (1 km at 100% + 1 km at 90%), 8 km in total, and add one repetition every two weeks. Keep the float at 88-90% until the 1 km repetitions come out even.

An advanced runner can build to 12 repetitions, or bring the float to 92-95% to get closer to race pace. With a 4:12 half marathon pace, 94% comes out at 4:27/km.

The progress rule is the same at every level. If your last kilometer at 100% feels as controlled as the first, add a repetition. If you finish grinding, repeat the session next week.

When to skip alternations

• In the first weeks of the plan: the general phase builds volume first.
• If you cannot hold 100% in the first kilometer of each repetition: lower it by 1% and repeat the session.
• One week before the race: the session costs too much freshness.

Reference paces (1:28:40 half marathon)

Your paces depend on your result. Work out yours in the calculator and build the full plan from the home page.