The aim of the study was to examine the effects of two different training programs (high-intensity-training vs. continuous endurance training) on aerobic power and body composition in recreationally active men and women and to test whether or not participants were able to complete a half marathon after the intervention period. Thirty-four recreational endurance runners were randomly assigned either to a Weekend-Group (WE, n = 17) or an After-Work- Group (AW, n = 17) for a 12 week-intervention period. WE weekly completed 2 h 30 min of continuous endurance running composed of 2 sessions on the weekend. In contrast, AW performed 4 30 min sessions of high intensity training and an additional 30 min endurance run weekly, always after work. During an exhaustive treadmill test aerobic power was measured and heart rate was continuously recorded. Body composition was assessed using bio-impedance. Following the intervention period all subjects took part in a half-marathon. AW significantly improved peak oxygen uptake (VO₂ peak) from 36.8 ± 4.5 to 43.6 ± 6.5 [mL.min^-1.kg^-1], velocity at lactate threshold (V_LT) from 9.7 ± 2.2 to 11.7 ± 1.8 [km.h^-1] and visceral fat from 5.6 ± 2.2 to 4.7 ± 1.9 In WE VO₂ peak signifi-cantly increased from 38.8 ± 5.0 to 41.5 ± 6.0 [mL.min^-1.kg^-1], V_LT from 9.9 ± 1.3 to 11.2 ± 1.7 [km.h^-1] and visceral fat was reduced from 5.7 ± 2.1 to 5.4 ± 1.9 (p < 0.01). Only the improvements of VO₂ peak were significantly greater in AW compared with WE (pre/post group interaction: F=15.4, p = 0.01, η² = 0.36). Both groups completed a half marathon with no significant differences in performance (p = 0.63). Short, intensive endurance training sessions of about 30 min are effective in improving aerobic fitness in recreationally active runners.