Hi, Rhiannon! I'm not familiar with any of the articles you've read; I'd be interested to read them myself. An important thing to keep in mind when reading any literature involving fitness is the source of the article: is it a solid, academically-oriented article or is it a popular trade article? I tend to be very skeptical of popular publications like "Shape" or the Weider publications group (Men's Fitness, Women's Fitness, Women and Men's Fitness, Squirrel's Fitness, what have you), and tend to pay more attention to scientific exercise journals from academic institutions and groups.
That said, I'll give you a somewhat informed opinion: the essence of the physiological "training effect" is: the body adapts to the demands placed on it. What was a challenging training stimulus (say, going into running after years of fitness walking; training with heavy weights as opposed to lighter weights) simply becomes less challenging as your body becomes more fit. To sustain a new level of fitness, indeed you need to maintain the level of the new training stimulus. Thus, your body does "require" more of the heightened training stimulus to stay at that level of fitness . . . but that's good news, isn't it?
You can increase the overall training stimulus usually in three ways or in combinations of these ways: increase the FREQUENCY of the training; increase the DURATION of each training / exercise bout; and/or increase the INTENSITY of each training / exercise bout. My own preference is to do extended exercise sessions 4 to 5 times per week (usually 4 times per week these days), and when I've gotten to the point where a challenging protocol feels easier, I prefer to upkick the INTENSITY of the routine within that schedule, rather than extend the duration or add in another workout session. And frankly, I believe I increase overall fitness levels by focusing on intensity rather than duration or frequency.
Just as an example, when I first got Boot Camp, those squat-thrust climbers, extended sequential power kicks and extended ice breaker sequences made me bleed through the eyeballs at first. Of course, that made me do the bloody routine at least once a week until it felt like work rather than death. Now, I feel quite comfortable with these sequences - they're not easy, but they don't bring me to failure anymore. The training effect in action . . . and I have to do all the Boot Camp cardio intervals at least once a week now or I get the heebie-jeebies. Same with Interval Max 1 - I have to do that one at least once every two weeks or I go into a mental / physical slump.
Don't know if that answers your question; please let me know if you have any others. And thanks for asking!
a-jock