I would just slow down and go with what comes. If you are timing these
3 day a week runs it can be defeating because it is hard to feel good all the time.
I would say you need to continue to run easy and when the good days come run a
bit faster but don't time them. Just move along at a decent tempo pace for 20 - 30
minutes, whatever, and call it a day.
The primary goal in building aerobic fitness is running as much as you feel you can
and leave something for tomorrow. Of course building to new levels requires adaption
and this is harder. If your tired all the time, then back off and stay at that level a
couple extra weeks. Honestly, everyone has a different definition of tired. I am tired
all the time. I just get out there and see how it goes. Usually, I am hitting the workout
load I want.
I still time runs (specific workouts) occasionally, but more than likely it's like yesterday;
8.3 miles, 1:10, 15 x golf course hill. If I know for certain I won't hit a timed workout
(insert any reason), I will substitute 25-30 minutes at a real good effort.
I believe you have leveled off because your working a bit harder at this point than you
should. It has an accumulative effect. Instead of days off, I would be finding a way to get
running about everyday no matter how slow the miles. As your fitness increases, so will your
ability to handle those miles at a bit faster pace. You can't force fit. This has never failed
me. Unless there was some other under lying issue.
Hope this helps.