I'm too tired to read the whole thread, but I have a Fitbit Charge HR, and even though it is a bit buggy (buy from somewhere with a good returns policy), it is great for pacing. First of all, it's counting steps. You set your "goal", which for us is more of an upper limit, and it buzzes when you get there. So if that generally happens late in the evening if at all, and one day it happens at lunchtime, you know to scale things back and start resting. You can also look at your charts, and if you notice that you have been doing a lot more over the last few days, build more rest time in.
Secondly, the HR function is brilliant. When I am performing my pouncing duties with the cat (my version of short, intensive exercise, and probably the closest to aerobic exercise that I get), I keep checking my HR, and if it goes to the 90s, I stop. When I am building up to have a shower, which is the most exhausting thing for me generally, I keep checking my HR, and if it is 90s or above, I stop and rest a bit. I have set my date of birth on Fitbit as 1900, so it has correspondingly lower target ranges for HR. This is just what works for me personally, it depends on what your HR tends to do, but it doesn't take long to get a sense of what works for you.
