You can google VDOT and VO2Max and find out everything about the math and the formulae behind these concepts and why they are so important for training, or you can simply do the following:
After a 15-minute warm-up, do an all out run for 30 minutes at a track and measure the exact distance (to the nearest meter or yard) covered in that time. The idea is to run as fast as possible while being able to maintain a constant pace for the entire 30 minutes (within a couple of seconds per lap). It’s not easy; you’ll hurt for the entire 30 minutes!
Then, go to www.attackpoint.org and enter the time and distance. It will calculate your VDOT number and the corresponding paces at which you should train on intervals, long runs, recovery runs, etc. The site will also give you the amount of time that you should spend training at each pace. Follow this training plan, doing recovery runs between hard runs, and you’ll see your performance improve.
Here’s how it looks for me: