>General Running>Make me faster - building a sub-20:00 5-k
When is this target 5k? A time frame would be helpful, but isn't overly necessary.
On the tempo runs, start by doing the first quarter easy, the second quarter a little harder, the third should be at least your current 5K pace, and the fourth throttle back.
After that, I would start doing speed work once a week. I would do ladders (400, 800, 1200, 1600, 1200, 800, 400), negative 800s (do the second 400 faster than the first), and intervals. Keep a tempo run a week in there too, and you might add some form drills (Heel kicks, High knees, etc.).
Also, do a time trial, on your own, maybe once a month to see where you stand. Do it on the same course every time (or the track).
* I'm not likely to consistently do more than 35-40 miles per week.
Why is this? If there is a way to bump that to even 40-50 mpw on a consistent basis you will probably see huge gains. In my own running, mileage over the long term seems to make the most difference. No other single factor is really a close second. I think if you have the time, the best way to improve is to increase your mileage until you find your physical limit, then back off to about 75-80% of that limit and run at that level consistently for a long time.
The Logic of Long Distance
I agree with the upping the mileage, but do so in a fashion that you're not gonna burn out over it.
As to your questions:
Yes, the tempo is done in place of a regular run, over that run's set distance (if you're going for, say, 5 miles today, make it a 5-mile tempo run).
but if you just got to 40-50 on a consistent basis and basically made 40 your normal minimum with an occasional week of 45 - 50 when you have time, I think you would see a huge difference after 3-4 months of that. It would give a better base to do bigger workouts come spring, too. .
It's not a bad thing if running a fast 5k is not the most important thing in your life.
To add something here.....
I have a friend, who is currently getting ready to taper for a marathon in which he plans on BQ'ing. He just did a 5K yesterday. He managed to destroy his previous PR on the same course (he did about 20 min, this time around he did about 18 and change). So, there's a lot to be said for having a lot of miles, as well as improving short distance times while training for a marathon.
To my mind, whether or not you'll be able to run sub-20:00 will depend as much or more upon your ability to run efficiently at a fast pace rather than the size of your aerobic capacity (not that these are unrelated). Drills and strides will take you a long way in this respect.
So, I would recommend that you try adding more intensity and drills to your schedule (particularly given how much you love 400's ). Also, half of your weekly mileage is coming in one run, and because that run is so long, you can't use it to teach your body to run FAST. If you're training for a 5k, I'd say you don't need any runs over 1:30, and you should run once or twice a week for 30-50 minutes at the tempo-ish pace that Scout7 recommends.
Your log indicates to me that you can get there--you've improved tremendously lately. Good luck!