Thanks to Lee for putting my mind at rest... and giving me the chance to refresh on the physiological side of this. Do read the document - because there is more in it than Lee was able to paste into a sensible post. I do want to interpret it though, and ask if anyone disagrees with my interpretation.... please correct me.
If we build up the availability of blood flow (by developing mitochondria) to our muscle fibres, they can use oxygen/glycogen for fuel.... but when there is insufficient oxygen flow for any given running pace, the fibres work anaerobically. They can do this, but produce lactate and tie you up pretty quickly.
We have 3 types of muscle fibre:- fast-twitch-white (sprint), fast-twitch red (intermediate) and slow-twitch. Your body doesn't even bother using the fast-twitch-white fibres unless you're running fast 80%VO2max... so you don't build up mitochondria around those fibres at slow speeds. On the other hand, if you do all your running fairly fast, you largely bypass your slow-twitch muscle fibre systems, and run with your fast-twitch red (and maybe white) fibres... and it is these that get trained up to get lots of mitochondria for good oxygen supply. The slow-twitch fibres will get some benefit... but definitely sub-optimal.
So Dan, if the theory is correct and applies to you... when you did all your training at 9:30/minute, you must have been running too fast to use your slow-twitch fibres to cope... so your body quickly bypassed them and recruited your intermediate fast-twtich-red fibres. Maybe the first few times you did this you added a few mitochondria to the slow-twitch fibres, but they were always quickly swamped, and your body would spend its resources on adapting the intermediate fibres, not the slow-twitch ones. So your intermediate fibres would have more mitochondria, but these are designed for intermediate running speed /distance.. not really designed for long distance running, so you're not going to see dramatic improvements in long distance running ability, using those - hence your performance plateaued.). So this seems to explain why your performance has reached 9:30 pace, but stopped improving.
When you run faster, you say that your fast-twitch muscle fibres are well-enough trained to let you go for a mile at a decent 6:40 pace... but not surprising, no further.
But this weekend, when you slowed down to 10+ minutes/mile on that last long run, you must not have been running fast enough for your "fast-twitch-red (intermediate)" fibres to be recruited to the task. Your body was looking only to the slow twitch muscles, and as we've discussed, those haven't been optimally trained for maximum mitochondria... they can't feed blood fast enough, so start to produce lactate... and you feel bad... and you need to slow to a pace where your mitochondria CAN get enough oxygen in there. (According to hadd, when you were struggling at 10:30 pace, if you had speeded up to 9:30 pace, you would have felt much much more comfortable, as this would have kicked your body into recruiting the fast-twitch-red fibres, which you know are well trained).
So, what next?... If my interpretations are correct, you need to build up your long SLOW run distances (perhaps at 11 minute pace based on what you said about slowing to 10-40). As frustrating as this is, think of this as concentrating on adding mitochondria to those slow-twitch muscle fibres. Then, maybe in 3(?) weeks time, you can ask your body to run at 10:00 pace, and it will (as usual) first recruit the slow-twitch fibres - but instead of quickly exhausting them, it will find that their newly improved blood supply allows them to sustain the pace without needing to quickly switch to those fast-twitch fibres. And if your slow-twitch fibres are now sustainably running