If there is “Physical Fitness Test” for the Officers and Soldiers of any Armed Forces of any country in order to determine the level of their fitness and endurance based from military standards, there is also some sort of “fitness test” for runners based from the distance of the road race they are going to participate or compete.
In the book, “Run Faster: From 5K To The Marathon” by Brad Hudson, he suggests that if a runner is following a training plan for a certain race, he/she has to schedule a “specific-endurance” test (spec test) in order to assess if there is an improvement towards his/her desired goal depending on his/her target finish time to an specific road race. To some coaches and runners, such term is also called as “time trials” which are usually done in an oval track or in a road race. Such test should be done at least every 5-6 weeks. However, running in a road race is a practical option for 5K and 10K runners who can run such distances every few weeks without overtaxing their bodies. As for the half-marathon and marathon distances, the only option for the runners to test their endurance and speed is to perform a very “race-specific” workout.
A good spec test for the 5K is to be able to run an interval run of 5 X 1K @ 5K pace with a recovery period of 2 minutes. The runner must be able to sustain his/her goal race pace throughout all the five intervals. Your ability to run within your goal race pace or how easily you could do this workout will determine your current state of your specific endurance for 5K.
Recommended spec test for the 10K is to be able to run an interval run of 5 X 2K @ 10K goal pace with 3-minute jog recoveries in between runs.
There are two methods to determine or conduct the spec test for the half-marathon. One option requires the use of a heart-rate monitor, however, I suggest the spec test without the use of a heart-rate monitor. It is done at the oval track by running at the fastest pace you feel you could sustain through an entire half-marathon. Run exactly 4 miles which is equivalent to 16 laps + 40 yards and stop after finishing the distance and use your time to calculate your pace.
In the marathon distance, the author does not recommend any effective specific-endurance test as the performance in such distance is almost always determined by what happens in the final 10K, after 20 miles of race-pace running are completed. Either you hold your pace or you fall apart.
In my experience, my finish time for the longest run in my marathon training plan, which is 32-km/20-mile run and done at least 6 weeks before D-Day, determines more or less my average pace for the marathon. The well-known “Yasso 800” does work with me, however, this is also a good tool/drill to determine your desired finish time in a marathon race. (Please refer to my previous posts on this subject).
Why do you think that most of the marathon training plans have a maximum distance of 32 kms/20 miles for their easy long runs? Because your coach or whoever prepared such training plan does not want you to pre-empt or experience what happens to your body while running on the last & remaining 10K of the marathon while you are on training. You have to experience such “feeling” or “hitting the wall” on the marathon race day itself. If you have that “warrior or hardcore” attitude, nobody is preventing you from running a 42K on your long steady distance run, weeks before your marathon D-day. If you are planning to be an ultramarathoner someday, a marathon distance is just your regular weekly weekend easy long run.
Reference: “Run Faster: From The 5K To The Marathon” by Brad Huston. pp. 112-115.