We did a similar study with sedentary females aged 20-40. 3 groups -- 1 walked, 1 alternately walked and jogged, and 1 jogged (all 3 groups for the same amount of time). Over 12 weeks the walk/jog group showed the most improvement in treadmill stress tests. Surprising until you realize that if a sedentary person is asked to jog 20, 30, 40, 45 minutes at a time they will go much slower than those who jog only 1 or 2 minutes at a time with equal walks between. I assume the same with soccer -- because there is intermittant rest you tend to go harder when you do run. Get the run (jog) only group in some kind of shape and in a given period of time they will get more out of it than intermittant exercise.