A Typical Hard/Easy week:
Sunday: Easy (Rest)
Monday: Easy (5-miler)
Tuesday: Hard (10-miler)
Wednesday: Easy (5-miler)
Thursday: Hard (8-miler; with some fartleks)
Friday: Easy (Rest)
Saturday: Hard (15-mile long run)
Mix and match. The schedule above has two longer slower runs, but one of those could easily be swapped out for a shorter harder run. It's entirely up to you. Bottom line, if you break up your week into different types of runs, whether they're really formal such as in the form of intervals, tempo runs, and long runs, or if you're just thinking "hard" and "easy," you'll begin to see improvement in your endurance and speed. Give it a try and let me know how it works!
Also another rule-of-thumb is to keep about 20-25% of your weekly mileage at speedwork, tempo-run, and/or race pace. So if your weekly mileage is 35 miles, then about 7-9 miles should come from faster runs.