Hey there! I'm pretty new to SP as well, there is soooo much info on here it can totally get overwhelming. But today is my 4th day ever in my life that I have consistently drank water and exercised a minimum of 10 minutes a day. I'm going out for my 3rd run tonight with the couch to 5k program and I am totally exhausted right now.
I think that if your diet is a little off on the day you run, or if you're not hydrated enough that can make the run a little tougher. Of course if your just plain tired that will do it as well! good luck on your next run! And thanks for adding me as a friend, I've added you as well! 2054 days ago