Interesting to read your post as that is pretty much what I used to try to do, and as you say, although I did improve I did feel burn out and then my times started to plateau.
From reading a few running books there are a lot of supporters of the principles of 80/20 training, Maffotone, which is basically spending more time running at a slower pace, in aerobic zones, which should enable you to run faster. If you read up on a lot of the best Kenyan runners and their training camps in places like Iten a lot of their time is running at a much slower pace.
Its only anecdotal but I spend more time running now at a slow pace, and I think it has helped and additionally, my recovery times are a lot quicker (so can add more runs) and, as an added benefit, enjoy my running a lot more, listening to music/podcasts etc. Pretty much every week I try to do one run where instead of looking at distance/time I follow my heart rate and aim to keep it below 140.
So varying your training should help, adding longer/slower runs and if you are goal orientated running a little bit longer/or further could keep the competitive edge to it.