Are you logging lots of miles but not seeing any improvement in speed or endurance? Come to find out it's not really the quantity of miles you log but the quality that count. If you're running the same 4-6 miles every day at the same pace, day-after-day, month-after-month, you're not challenging your body. It's acclimated to this pace and distance. Kind of similar to lifting weights in the gym. If you do the same routine over and over, you begin to plateau and see no change. You have to keep your muscles guessing what's coming next. Same thing occurs in running. Dare I say it....try running less but make the runs you do count.
Here's a sample week:
Monday: easy 4-6 mile run
Tuesday: speedwork—track repeats or fartleks (about 3 miles total)
Wednesday: chill or crosstrain
Thursday: tempo run (begin run with a slow 10-minute warmup; then run at least 25 minutes at just below race-pace speed; finish with a 10-minute cooldown)
Friday: easy 4-6 mile run
Saturday: long run (8-11 miles) at an easy pace (note: if you're training for a marathon, your long runs will increase in length and go beyond 11 miles)
Sunday: chill or crosstrain
Feel free to modify the schedule to fit your needs. Many runners do their long runs on Sunday. That's fine. Just try to alternate easier days with the challenging days. I used a similar schedule to the one above and finally achieved a sub 20-minute 5K and a 1:30 half marathon. Everybody's different and results will vary, but the bottom line is, if you focus your runs (each one having a specific goal) you'll definitely see good results. Give it a try!