curious little boy wrote:
The heat just sucks the life out of me.
The heat is another stressor, similar to altitude. Your cardiovascular system is working harder, pumping more blood to the skin for heat disipation, and thus less to the muscles, resulting in slower pace. But your heart is working quite hard, and getting benefit, without the excessive pounding on the muscle structure from the slower pace.
When the heat breaks, you will feel like you are shot out of a cannon, as your cardiovascular system will be MUCH stronger, and that blood will now be available to the core muscles.
So, just keep at it, stay hydrated, and accept the slower pace. You will still get the benefit of the work effort, just from a different angle. Pseudo-altitude training seems to be a good analogy.