Hello. So just finished w2 :). But a little confused. The first two runs this week were surprisingly comfortable. I felt good running and was feeling confident about w3. But today's run was awful. I didn't go any faster, had had a rest day in between but had to really battle to keep going for the 90 seconds and had nothing left at the end. Now feeling less confident about next week. Is it normal to suddenly find runs difficult like this?