To improve in distance running, there will come a point where your normal 4 to 5 mile run will not make you any faster, you will need to do some sort of speed training. Speed training consists of three main types of runs: Fartleks, intervals, and tempo runs. Each run has a different purpose, but all three combined are used to increase speed, ability to sustain a quicker pace for longer period of time and improve mental toughness.
Fartleks (or speed play) are specific amounts of time run at a specific effort with a specific amount of rest in between (ie. 5 x 2 min. with 1 min. rest/jog in between). These runs aid in your ability to run a quick pace when tired and to deal with pace changes that happen in a race.
Intervals are typically done on a track and are repeats of certain distances in specific times with specific amounts of rest in between (ie. 4 x 400m in 75 sec. w/2 min. rest in between). Intervals are usually run at a quicker pace than Fartleks and are designed to increase overall speed and get you comfortable with running while fatigued.
Tempo runs are typically longer distances or time run at a quick, yet sustainable pace. This type of run is designed to increase mental toughness and your ability to sustain pace when tired.
The question is, how fast should your be running each interval, repeat, or tempo? If you have the knowledge and time, you could get specific and come up with paces or times you should aim to be at in order to help reach your intended goal. However, these times would change as you improve and your goal changes. If you want to go the route of having specific times or pace to train at, I would recommend getting a coach to help determine the paces and times specific for you.
I want to share a much easier way for determining how fast you should be running each of the three speed runs - its EFFORT. 100% effort would be running as fast as you possibly can, giving it your all - you should not be able to talk while running at this effort and you will not be able to sustain it for very long. 90% effort is slightly under “all out” - you should still not be able to speak and it will be a little more controlled. 80% effort is more sustainable and you should only be able to speak a few words at a time while running but not full sentences. 70% effort should feel comfortably hard - meaning it’s just fast enough to where you feel a little uncomfortable but with focus you can maintain that effort for a while. You should be able to speak a full sentence while running at this effort but not carry on a conversation. 50-60% effort should feel comfortable and you should be able to carry on a conversation will running.
When running Fartleks, anything ranging from 1 min. to 2 min. sets should be run at 90% effort. Anything from 3 min. to 8 min. sets should be run at 80% effort. Then from 9 min. to 15 min. sets should be run at 70% effort. An example Fartlek workout would be: Warm up by running 1 mile. Then do 1 x 9 min. at 90% effort followed by a 2 min. jog. Then do 2 x 5 min. at 80% effort followed by 1 min. jog between each set. Then do 3 x 2 min. at 90% effort followed by 1 min. jog between each set. Cool down with a 1 mile jog. Use this type of run 3-4 months prior to your goal race doing at least one a week.
When running Intervals on a track - distances from 100m to 200m can be run at 100% effort. Distances from 400m to 1000m can be run at 80-90% effort. Then, distances from 1200m to 3200m can be run at 80% effort. An example Interval workout would : Warm up by running a mile. Then do 1 x 1600 (1 mile) at 80% effort followed by a 2 min jog; then do 2 x 800m at 80% effort with a 1 min jog in between; then do 2 x 400m at 90% effort with a 1 min jog in between; then do 4 x 200m at 90% effort with 30 sec jog in between. Cool down with a 1 mile jog. Introduce interval runs into your schedule 2 months prior to your goal race. You can do 1-2 interval sessions per week depending on how much you want to do and how your body feels.
For Tempo runs, you should run the majority at 70% effort. Aim for 15 to 30 min. runs and you can even build to a 1 hour long tempo run. If you want to practice running your second half quicker than your first half, increase the effort of the second half to 75 - 80% or to where you can speak a few words but not full sentences. Introduce Tempo runs into your schedule 3-4 months prior to your goal race, doing one once a week or every other week, and continue doing them up to two weeks prior to your goal race.
Focusing on effort vs. specific time will make speed training less complicated and help you become more intune with your body. Now, get out there and let speed work take your running to the next level.