The efficacy of an 8-minute field test to prescribe exercise intensity and assess changes in fitness was evaluated before and after 8 weeks of indoor cycling, and the results were confirmed by laboratory assessment. Changes in maximal steady-state power (MSSP), power at lactate threshold (PTlact), maximal power (Pmax), and maximal oxygen uptake (JOURNAL/jscr/04.02/00124278-200702000-00047/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max) were measured on 56 participants (20 women, 36 men; mean ± SD. 46.5 ± 10.0 years) who completed 1-hour, biweekly indoor stationary cycling classes on their own road bike outfitted with a Power Tap Pro power meter. The MSSP was defined as the average power during an 8-minute field test, which was administered at the beginning (pre) and end (post) of the training intervention. Individual training ranges were calculated from the pre-MSSP in accordance with Carmichael Training Systems. Laboratory assessments of PTlact, Pmax, and JOURNAL/jscr/04.02/00124278-200702000-00047/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max were made on 24 of the participants the same weeks MSSP was evaluated. After training, MSSP increased 9.2% (195.4 ± 56.6 vs. 213.8 ± 57.2 W; p < 0.05), and PTlact increased 12.9% (178.3 ± 47.1 vs. 201.5 ± 47.6 W; p < 0.05). The MSSP was −7.5 % higher than PTlact. Pmax increased −6.7% (315.2 ± 65.1 to 336.5 ± 65.9 W), and JOURNAL/jscr/04.02/00124278-200702000-00047/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max increased −6.5% (46.2 ± 10.7 to 49.1 ± 10.5 ml·kg−1·min−1). The MSSP and PTlact were highly correlated (r = 0.98) as was MSSP and JOURNAL/jscr/04.02/00124278-200702000-00047/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max (r = 0.90). The results of this research indicated that (a) the field test is a valid measure of fitness and changes in fitness, (b) it provided data for the establishment of training ranges, and (c) a biweekly power-based training program can elicit significant changes in fitness.