Completed W6R2 today and found it rather tough. A few days ago I did 20 minutes straight and it felt so much easier than 2X 10mins. I got through it even though the 10mins felt like an eternity. I guess as of today I am done with intervals, that's kind of a scary thought. I'm finding that as I'm running longer, my knees and left ankle are a little tender, I wouldn't call it pain but mild discomfort and it's not usually while I'm running but later. Does this sound normal or something I should be concerned about?