A big part of developing as a runner is finding out what works for you, sometimes the hard way. Some people run well off high miles, others just break down. Theres arguments that support vitually every kind of training known to man but you have to find what works best for you.
Also keep in mind that even elites go through slumps.
Your slowing times could be the result of a number of things from medical to physiogical to psychological. Do some tweaking in your routine and see what happens. You said you do no speedwork; throw some in. Is the mileage killing you? Back off for a couple of weeks. Yes, for many runners more miles does equal faster times but not for everyone. Theres also the types (me included) who tend to run sparkling workouts but hardly any faster in races because they either choke or are running workouts at a much faster pace than they should. If you want to keep the mileage up, great - but slow down. As much as we all tend to dislike speed if you don't train your body to run sub 5:30 pace strength via mileage along won't always do it. Add something as basic as pickups during your easy runs or throw in a tempo run every week or two. Again, we all have various kinds of advice for you, and most of us have been in your shoes however you have to find out what works for you. It may take some time but I'm sure you'll find it well worth it. Hang in there & good luck.