Hi Helen,
Its not uncommon to feel it for first two miles or so most runners, including myself, feel that way. Although I have heard it said that its because you dont warm up properly.
The reason your struggling to improve or finding it hard at the moment is usualy down to one of two reasons. 1, Simply that your unwell. or 2, and most common, you are doing to much for your fitness levels.
Your body will tell you in a shot if its not up to the level your doing and I suspect thats whats happeneing. Slow it down for a while but do the distance some days and speed it up but dont do the distance others would be my advice. Build your stamina.
I run six miles a day six days a week and I can tell you my best run is always without exception the one after my rest day so I think that says something.
The important thing is not how far or how hard you run, the important thing is that you get out there and do it. Dont beat yourself up about times or distance I know alot of runners who didnt see any great improvment for two years.
Keep it up and well done.