Row 1000m
…for time. Snatch weight should be light enough to complete the first 10 reps unbroken.
Goal: artificially (but rapidly!) elevate the heart rate with the snatch,and then operate at a high wattage output for 4-5 minutes without stopping (that's the row part.) This will do more to develop your 'cardiovascular capacity' than any 'base-building' run ever could!
From the SICFIT site (dedicated to the Sport of CrossFit): AMEN.