Many runners plan their long slow distance (LSD) running to cover a certain number of miles or kilometers. Some runners, however, plan their LSD running to cover a certain amount of time. As we plan our running, we need to decide if we will plan distance or time.
In general, runners run two two types of routes: loops and out/back. Because loops are a fixed distance, runners can't change the distance they run, and they run for that distance, even if they keep track of the number of minutes they run. However, in running out to a particular point and then running back to the starting point (out/back), runners have control over the distance to the turnaround point, and they can vary that distance to meet their needs. Measuring the length of a run by time instead of by distance is feasible, and the rest of this post concerns out/back.
Advantages & Disadvantages of a Distance BaseWe are conditioned by society to think of distance in miles or kilometers. Planning a route in distance has the advantage that runners always know how far they have gone. This type of planning has the disadvantage that runners can easily run too far for their present condition. Our body condition, our ability to comfortably run a particular distance varies from day to day. However, many runners don't change their distance goals based on how they feel. This means that if runners become tired before reaching their goal, they may push to reach their goal and run past the point of comfort. In doing this, they become really tired and increase their risk of injury.
Advantages & Disadvantages of a Time BaseRather than run a particular distance, some runners run a particular time. This means that on days when they feel peppy, they run faster and thus farther in the amount of time of their run. And, on days when they feel tired or have hills in their route, they run slower and thus do less distance. This means that running for a particular number of minutes gives them an automatic control on their running such that they are less likely to overextend themselves (assuming their time-goal is realistic and not an over-extension in and of itself). This control is important, because it reduces the risk of injury. It also means that as they get stronger due to a larger base of long slow distance (LSD), they will automatically increase their miles while still running the same time.
What I just said, about a goal of time instead of distance, is true if the pace out is the same as my pace back. However, if the paces aren't the same, the situation is different. If the out-pace is slower than the back-pace, the runner will return to the starting point before the time-goal is met. The runner will have to decide if he/she wants to stop running before the goal is met, or if extra running or walking should occur until the goal is met. If, however, the back-pace is slower than the out-pace (probably the most common situation), the runner will meet the time-goal before reaching the starting point. This means the runner will have to decide if he/she wants to push by running the remaining time, or to jog or walk that time as part of the cool-down.
In running out and back, the runner can select the turnaround point. If the pace out and the pace back are expected to be about the same, the runner can choose a turnaround point that is at the center of the time-span of the run. However, if the two paces are expected to be different, the runner can choose a turnaround point that is not at the center. The part of the run that is expected to have the fastest pace should have the smaller number of minutes allocated to it. By doing this, the runner can expect to arrive at the starting point close to the time when the run should end.
If a person runs the back or return part of the run slower because of getting tired, he/she should abort the run after meeting the time-goal. Continuing to run when you're tired increases the risk of injury. It's difficult for many people to abort a run rather than pushing through to meet their goal of a certain distance. Using a time-goal makes it easier for the person to abort the run when tired since the time-goal has been met. Thus, an advantage of running for time when doing out and back is an emotional advantage -- it is easier to abort the run when you're getting tired if you've met your time-goal but are not back to the starting point.
There are disadvantages of a time base for LSD. The main disadvantage is if one is following a training plan that specifies distance. Another disadvantage is if a non-running friend asks how far they are running, and the runner answers, "An hour"; the friend probably won't know how to relate to that response. You can get around this disadvantage by knowing the approximate distance of the run and answering in miles or kilometers (the friend probably doesn't want an exact figure).