I've been a jogger on and off for my whole life, but never one for endurance...so last week Friday (and other times throughout the year) when I experienced an extreme readiness to work out I pulled off 1/2 an hour interval training, heart rate around 160 the whole time, feeling great. I felt like I could breathe. This was such a huge shock for me because normally when I interval train, I can only last about 12 minutes and my heart rate stays around 180. This is hard for me to figure out...my diet stays mostly the same, though I could understand if the amount eaten has anything to do with it. The day I ran well I noticed I ate a tad more than usual. Have any of you experienced this? Why would some days be so different than others?