I'm not suggesting you retire, just re-evaluate. I suspect not many on here have had to call out a MRT once, let alone twice as it seems you have? You say you limit your LDW walk sectors to around 10 but the example you gave was a 15 mile leg in the heat, across quite tough terrain and you have a heart condition. You got around 10 miles into that walk then made a rather dubious call to press on when you weren't feeling too good, resulting in a MRT call out.
Can you not see that suggests you're taking on more than you should? I wouldn't want to see anyone stop walking, but it should be in line with their capability.