I honestly think planning the walking and the rests is part of the problem. if you tell yourself in advance that you'll walk amount x and rest amount x, you are not listening to your body and you inevitably overdo it. I really believe we need to do as much as we can to prevent deterioration. But that's hinges awfully on much and can.
You don't know the dimensions of much or can in advance (indeed often you don't know on the day, during, or even in the immediate aftermath). So it's about being opportunistic. Cautious, but opportunistic. Go very easy and grab exercise when and where you can.
I'd also suggest walking for 14 minutes non stop is too much if your total envelope is 26 minutes. Set out in a direction where you can sit down much sooner if necessary. (I know the location of every low fence and park bench within 2km of my house!)
Haha...I can definitely relate to knowing where every bench/wall is just incase I need to sit down!
It's hard to say what my energy envelope is, you're right about the 'much' and 'can' variables. For example, i'd say yesterday was a bad day for me and after the walk I laid down for 30 minutes and started to feel really fatigued, but oddly I still managed to get up and drive to the gym later in the evening to meet a friend (I didn't workout as such, just did some stretching and light abs). Astonishingly I woke up today feeling significantly better than yesterday.
The thing that actually seemed to give me a lot of energy back was getting up from being laid down, having a shave and putting my gym gear on, so even though my perceived energy was very low to begin with, it actually improved greatly once I got up and moving. I have POTS too so perhaps dysautonomia/circulation issues contribute greatly to my fatigue perception, skewing it somewhat. Subsequently my energy envelope/PEM threshold could be slightly bigger than my perception.