I do have one more question - should my balance between easy and not easy runs remain the same as I increase the miles?  Of course increasing the proportion of speedwork while also increasing mileage would be bad, but is it ok to keep a balance of ~50/50 as I'm working my way upward? 

I can only speak for myself and have only been running four years but the time I spend actually running fast per week is nowhere near 50% of my time spent running.

My speed work is based on a percentage of my overall weekly mileage and goes something like 3% for "R" pace intervals (which is around mile pace, 5% for VO2 max intervals (around 5k pace) and 8% for my tempo runs and intervals (right around 10k pace).

At 45 miles a week or so that comes out to around 1.5 to 2 miles at "R" pace, 2 to 3 miles at VO2 max pace, and 3 to 4 miles for tempo pace.

That I only do two speed work sessions per week means I am only doing two of the three during any given week.  Another thing to consider is that your long run is considered a quality run as well, so for me that's three quality workouts per week.  Although I sometimes will drop a speed work session and run easy because I am not as young and pliable as the people most plans are written for. 

Sorry, I probably wasn't clear - I meant that my long run + my speed work day (including the warmup and cooldown and recovery intervals if applicable) are around 50% together.  The rest of the week is just easy runs for the other 50%.