The purpose of this study was to develop a standardized and externally paced field test (15-m Incremental Shuttle Walk and Run Test [15mISWRT]), incorporating an incremental and progressive structure, to assess aerobic fitness in middle-aged adults. 68 middle-aged men performed three tests in random order between one to two week intervals: 15-m ISWRT, cardiopulmonary exercise test (CPX), and 1500-m fast walk. Variables evaluated were 15-m ISWRT performance (distance completed), VO₂max measured by CPX, 1500-m fast walk performance (walking time), and HR response in 15-m ISWRT and 1500-m fast walk. Validity of the 15-m ISWRT was tested by comparing the associations among the 15-m ISWRT performance, VO₂max and the 1500-m fast walk performance. Changes in HR response during the 15-m ISWRT and the 1500-m fast walk were also compared. Correlations between each variable were as follows: the correlation between 15- m ISWRT performance and VO₂max was very high, r = 0.86 (p < 0.01), the correlation between the 1500-m fast walk and VO₂max was r = –0.51 (p < 0.01). HR response during the 15-m ISWRT gently increased initially, whereas HR response during the 1500-m fast walk rapidly increased from the start. In conclusion, our findings indicate that the 15-m ISWRT is valid and safe for evaluating VO₂max in middle-aged adults.