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Allie Ostrander takes on one of her toughest sessions of the training block: 4 x 1 mile with recoveries, capped with 5 strides — all at 5,300 feet in Boulder, Colorado.

The splits: 5:02, 5:01, 5:00, and a 4:57 to close — with her heart rate peaking at 186 on the final rep. Every number on screen is live from her COROS watch.

THE WORKOUT

4 x 1 mile @ 4:57–5:02 (2:30–3:00 recovery)
5 x 30-second strides
Location: Boulder, CO (5,300 ft)