For amusement, I did a little math on the "Expanding the Envelope Story".
If you start at 15% capacity and increase your activity "a little bit" (I used 5%) every week, you could be at 100% capacity in a mere 40 weeks! If you take into consideration bad weeks and other down times so that your average "little bit" increase is only 5% every other week, it will take you 78 weeks (18 months) to be back to full capacity.
Now why didn't I think of that? Here I have been ill 11 years when a bit of gentle increasing my activity a bit would have had me back to 100% in a mere 18 months. Silly me.
Let's see... if you are a severe patient who can only walk about 20 ft (7 m) without PEM, and you increased your walking by 2.5% per week (since you're unlikely to be well enough every week to increase by 5-10%), after a year, you'd be able to walk 70 ft (21 m). Hurrah! That's valuable.

But then, I'm not sure how you are going to walk only another 6 inches (15 cm) every week. What is that, half a step? So increase one step every other week? No, wait, that would be a half step out and a half step back. Well then, I guess you increase by one step in each direction once a month. Sounds like fun. But not like it's going to get me very far... another 12 steps across the house in a year.
Okay, so maybe this doesn't work for severe patients (surprise, surprise). I'm not sure how Elena got from 15% capacity (that sounds bedbound to me) to 85% capacity with her increased walking program. Surely she couldn't have been walking 300 ft (91 m) when she was at 15% capacity. She must have started her walking program when she was much more functional. I don't know how she got from 15% capacity to able to walk the length of a football field. That wasn't in the story, but might be helpful information for those who are not able to start walking at 100 yd (91 m).
I also did some math on Elena's increasing walking story. She regularly, if she had a good day, increased her walking distance by 10-20 feet. By my calculations, that means if every week was good, she increased her walking distance over the course of a year from 100 yd (91m) to 350 yd (325m). If all days weren't good (and who has all good days with ME/CFS?), then it might have taken 2-3 years to improve that much. I can see that could be considered a beneficial improvement since it's the difference between shopping at a convenience store and shopping at a small grocery store, but it probably took 2-3 years to get there, walking every day. Two to three years of GET walking every day to gain 250 yd (230 m) of walking distance. That's some serious determination.
I wonder if the same can be achieved with ADLs for those of us who couldn't waste energy walking because we couldn't even cook and clean for ourselves. Could I increase my ability to wash the dishes by washing another quarter of a dish? Increase the amount I vacuum (hoover, for you Brits) by 10 sq ft (1 sq m)? Increase my shower time by 1.5 minutes every week? It just doesn't seem to translate to anything practical. If it's all about walking increasing strength or stamina, then it sounds more like you're dealing with OI or deconditioning, not ME. Our inability to function is not about weakness, really, it's about inability to produce energy and/or a pathological physiological response to exertion.
Is it that this program is effective for chronic fatigue (the symptom) rather than ME? After all, exercise does appear to be helpful for a number of conditions that cause chronic fatigue -- OI (in the absence of ME), cardiac conditions, fibromyalgia (in the absence of ME), deconditioning, etc.
While I'm straining my brain trying to figure out GET (even the gentle GET described at CFIDSSELFHELP)...
Why don't healthy people use GET to go from 100% capacity to 500% capacity by increasing their activity 5-10% per week? Surely the premise is that an individual's body does not have a physical limit or PWME would not be expected to increase their functional capacity 10x just using GET.