There are several ways of judging what pace you are running at. One is obviously wearing a GPS watch but try to get used to "knowing" and “judging” what each of these types of running feels like.
You will hear some coaches saying “Run at this pace or that pace, for example, 10k pace etc but this can be tricky to judge.
If you are asked to run at any given distance "Pace" during training, this means the pace if you were to run that distance today at your current fitness level. It also means the mid race pace e.g. mile 3 of a 10k and not the start or the (sprint) finish. In training, try to think of "pace" also in terms of effort level as you will be more likely to achieve the session goal. Getting used to what your effort levels are at any given pace is really valuable and will mean you will always train and perform right on your current fitness level.
So listen to your body and judge the level of intensity not (always) the pace.
Rate of Perceived Exertion (RPE)
This is your perceived effort and is an easy way to judge how hard you are working. Your effort is the most important thing, not the pace/speed. Imagine a scale of 1-10, 1 is on the couch watching TV at home and 10 is running as fast as you can. The club coaches deliver sessions that are designed to target specific thresholds. Perform these workouts too fast or too slow for your own current fitness level and you miss out on the intended benefit.
See the chart below as a guide and learn to judge your own required effort for each given pace.
Easy jog just to ease you in and out of workouts. Usually 10/15 minutes in duration so you can gently warm your body temperature up, get the blood flowing and prepare your mind for the activity ahead. Very easy normal chit chat. RPE 1-2
Easy or Recovery Effort - These runs allow you to see improvement without breaking down. These should feel very easy and relaxed. Enjoy the scenery. Breathe easy and you should be capable of holding a conversation. RPE 3-4
Steady Effort -These runs build your aerobic base that acts as the foundation for the rest of your training. Conversations are possible in sentences rather than a long gossip. The one thing with these runs is not to go too slowly. While running at this effort level, you are improving your Aerobic Threshold and this will effectively represent your Marathon Pace. RPE 5-6
Tempo Effort - The key for you to stretch your physical boundaries. These runs are controlled discomfort. They require concentration. You can utter a word or two but no more. As you get fitter and stronger you will constantly push your boundaries on these runs. They never get easy you just get faster and cover more distance. While running at this effort level, you are improving your Lactate Threshold (The point at which your body is happily removing lactic acid at the same rate you are producing it). It is the pace / effort level you can maintain for about an hour. Long intervals at this effort level are also useful as a way to increase the overall duration. This pace is approximately 10-15% slower than Fast Effort (below) so it’s quite subtle and important not to run this effort too fast. RPE 7-8
Fast Effort - No need for a description. While running at this effort level, you will be improving your Alactic Threshold (The point at which your body is unable to remove the lactic acid quite as quickly as you are producing it). Training at this level will involve maintaining the effort for a much shorter duration and/or shorter reps with longer recoveries in between. No talking. RPE 9-10
Daniels Running Formula - Jack Daniels
Another option for measuring your pace is to use a recent race performance to calculate your optimal training paces. This will show you what your easy pace, marathon pace, threshold paces etc currently are. Regular benchmarking is a good way for you to always know exactly where you are in terms of your individual fitness level. It will allow you to train to your fitness level and ensures you are always training at the right level, not to a goal which may not be realistic. This is how you get fitter.
Here's a link to a calculator on the Runbayou website and once you've had a look at that, have a look at the rest of the website. Daniels Running Formula is a good read if you would like to research this theory further.
Here are a couple of really good Lectures given by Jack Daniels, explaining his philosophy. These lectures are also available on UKA Coaches website, UCoach and so his theories are very much validated.
Greg McMillan at McMillan Running also has another good calculator which not only calculates your training paces but also provides the opportunity for you to see where you are in terms of your goal - Click Here