How to improve my running fitness
This question is a little too general to answer accurately. So we'll give you the challenging response rather than the benign or stock answer. "Fitness" is divided into two categories: endurance and speed. To increase your overall "running fitness" means you have to do work in each of these two broad categories.
To increase your endurance, you need to increase your volume. That puts your cardiovascular system through sustained effort. It is seldom wise to increase your total weekly distance by more than 15% per week. If you're running 25 mpw, that would mean adding about 4–5 miles for the total week. That's plenty for starters.
Some choose to do this by increments, while others add onto their "long" run each week. Your goal might be to get from a 5-miler to a 10-miler over a period of 8–10 weeks.
On the speed front, running fitness is achieved best through controlled interval training. A running track is your best tool for this. A great workout is to do 8 X 400 at 20% below your average running pace. So if you do 8:30 miles you would run your intervals at 8:00 pace, or 2:00 per lap. Keep this up each week and start with a full lap jog the first week for recovery. Then cut down your rest until you're jogging just a half lap. When you've achieved that pace with less rest, you can do one of two things: drop your pace or increase the number of intervals. If you're looking to race shorter events such as 5Ks, then drop your pace. If you're going longer distances such as a half marathon, then you add intervals. The most 400s I've done in a single workout is 28.