One thing I have learned with running. It's just as much a mental exercise as a physical one. You need to believe in yourself, or you make it a huge battle. If you convince yourself that you can do it, then the run is much easier.
When I first did my for 20 min run (end of wk 5 I think), I was so scared that I was not up to it, so had a horrible start to the run. Once I started to think, actually I might just make this, then the whole run changed and I did not find it anywhere near as difficult.
Even now a 20 min run can be a struggle if I am not in the right frame of mind. Weird really!!!