So I've been working on increasing the level of intensity that I can work at. This means that I can function at 90% of my max heart rate.
However I'm finding that my endurance has not increased very much and it's possibly why at altitude that I quite literally run out of oxygen and my muscles stop functioning very well. I have always thought it was the altitude since it always happens at particular elevations and that might have something to do with it, but it could be that I can only maintain working at 90% for so long.
Aerobic training (the kind where you can continue on indefinitely) is more around 85% of your max heart rate so I think I need to spend some time forcing myself to stay at that rate which is surprisingly hard when you have the illusion that you can go faster/harder.
One reason it's taken me this long to figure this out is that I only recently addressed the issue of my getting too hot. When I over heat I tire very quickly and start to trip over my own feet. I finally figured out if I wear a one piece bathing suit with a mostly open back, then the air is able to move around my back and keeps me cooler (I wear shorts too to keep the ridiculous look under control.) During the very first workout I was able to go longer, but still ran into a wall at just past 2 miles, so now I can start to work on the endurance angle more.
This makes me think that when I climb Mt Hoffman I should wear a heart rate monitor (what a great tool), and try to stay at 85% and see if I have the same altitude issues.