I think one of the genius moves by Lydiard was that he was very vague when talking about paces. There is a very good reason for that. There is no formula, whether it be produced by race times, heart rate, the phase of the moon, barometric pressue, or some other method, that will give you the right pace for every day. What seems vague to most people is what I view as him saying what I simply call “listen to your body”.
Then you are not running at BAE. You are running too fast. The simple solution is to slow down.
No, you slow down to a pace that you can hold or at least come relatively close to on a regular basis. On those days that you are feeling better than expected, maybe you speed up a bit. If you are feeling worn down, slow down as much as needed. However, your daily runs should be in a pace range that you could hold every day while running 1-2 hours per day (read on for more specifics on how long to run).
In this case, your 6:50 pace is too fast. Slow down to a pace range that you can do on a regular basis. There’s nothing wrong with running 7:30 pace or slower. Heck, quite a few of my runs recently have been in the 7:30-8:00 pace range but I know they will get faster as my fitness improves.
I’d say that’s a pretty reasonable reading but I would say the last sentence is a bit off. If your legs are feeling sluggish and you can not continue running at roughly the same paces, chances are you ran too fast. Do what you said in order to get your legs feeling good again, then adjust your efforts as needed in order to avoid a recurrence of that situation.
Lydiard’s suggestion is to run a set amount of time every day at BAE. Off the top of my head, I believe it’s 90 minutes Monday, 60 minutes Tuesday, 90-120 minutes Wednesday, 60 minutes Thursday, 90 minutes Friday, 120+ minutes Saturday, 60 minutes Sunday. These times should all be in one run per day at BAE effort. This is your baseline training. If you can not do runs of the prescribed duration for the given day, you are running faster than BAE. On top of this, add as much running as you can at whatever pace you want to run, just make sure these runs do not affect the duration of the key runs, although they may have a slight effect on the paces of those runs. These secondary runs are where “run as slow as you like” would come in.