Karaaaa

I just completed day two of week four and it is so hard! I found the rest of the weeks difficult but not like this. I am able to do the first eight minutes of running with out any real struggling. But when it comes to the last eight, I'm exhausted. Towards the end of the three minute interval is where I really begin to struggle, but the final five seem impossible and I walk most of it. It's really frustrating me so much! Any help you guys could give me would be amazing!