For these kinds of problems, you need rest, not harder running. In fact, the day after a harder effort is a good rest day. Aerobic exercise, biking, rowing and swimming are great for giving you additional strengths in different muscle groups than those developed by running and they will also give you protection from injuries. By avoiding injuries you can be consistent in developing your abilities and reach the starting line with plenty of confidence. Add Some Weight Training Weight training is also effective in this way.
Every bit helps.
You will be increasing muscle cell size and decreasing fat cell size. Get a Little Help from a Friend A weekly program where you can count on a run with a co-worker or friend is a great boost. You become a member of a team. Don't worry if you miss a run during the week, it's not a big deal. But if you are looking to increase your speed, the longer runs and the harder speed workouts and the ones that do the most for you.