My first setback

Yesterday I did 8min run/2 min walk x3 and it felt hard.  I decided to run today, and at first it was challenging, but then didn't feel hard.  I made an off-the-schedule decision to see how far I could run in one length, thinking I could do 15 minutes or possibly 20.  I made it to 13 and couldn't go any further. 

Then, I decided to do 8 minutes x2 after that, but made it to 4 and then 2, with my legs shaking.  I guess I pushed them too hard!  I'm going to take tomorrow off, and go back to 8 min on Monday.