You need more than one-off snapshots of your fitness levels to assess your progress properly. Races and time trials are reasonable guides to how things are developing, but they vary too much to be really useful. Races also rely on maximal effort, which is greatly influenced by the ups and downs in your motivation.
Its just as bad to try to monitor your fitness using a stopwatch on steady training runs. Everyone has fallen into the trap of checking their splits at almost every corner in a training session. The problem with this is that you end up racing against yourself and running too hard to get consistent results.
You might not be surprised to find that your heart rate monitor is the answer. If you use it to control the work you do, you can assess your progress very safely as part of your routine. Its easiest on a training course you use regularly, and though you could assess yourself each time youre there, its best to do it every three or four weeks.
Choose a heart rate that will keep you comfortable throughout the run. A good benchmark is a rate similar to the one you settle towards on your longer runs. If you have a monitor with upper and lower limit alarms, set the upper limit to this level. All you have to do then is time your run around the course, making sure you dont exceed your upper limit. If your monitor doesnt have an alarm, youll have to keep a close eye on it. Whichever method you use, youll end up with a record of how long it takes you to run a set course below a particular heart rate.
In this way, you can assess your fitness without having to run flat out. The next time you run the course, stay within the same heart rate limit and then compare your times. As your fitness progresses and your running becomes more efficient, youll find that you can run a little quicker at the same heart rate. The beauty is that by running in a controlled fashion you wont be wearing yourself out in training.
You can also monitor your progress more regularly by noting how long it takes you to recover after each session. Typically, you would record the time it takes your heart rate to drop to 120, or 100 if your training heart rates are usually much below 150. As you get fitter, your recovery time will drop, though naturally it depends on the intensity of the work-out you do. It will also be affected by the duration of the run and the temperature.
If you record your recovery time alongside other information in your training log, youll soon get to know what your recovery times should be for different kinds of work-out. If you find that the recovery time increases sharply (say 10-20 seconds or more), you have either overdone it, or picked up an infection. In either case, take it easy when you next work out.
There will always be times when illness, injury, overtraining or fatigue hit. Youll find that as well as your recovery time increasing, your speed for a given heart rate will drop but your HRM should help to stop you running too fast during recovery.