Find your running pace per km/mile from distance and time.
Pace is time per unit distance — minutes per kilometre or per mile — while speed is distance per unit time. They describe the same thing inverted, which is why a lower pace is faster while a higher speed is faster. Runners and swimmers use pace because it maps directly onto race planning: if you know your target pace, multiplying by the distance gives your finishing time without further arithmetic.
The inverse relationship has a consequence worth internalising: equal improvements in pace are not equal improvements in speed. Going from 6:00 to 5:30 per kilometre is a much larger gain in speed than going from 4:30 to 4:00, even though both are thirty seconds. This is why pace improvements get progressively harder as you get faster, and why comparing two runners' improvement in seconds per kilometre flatters the slower one. When converting between minutes-per-kilometre and minutes-per-mile, remember a mile is about 1.609 kilometres, so a mile pace is always the larger number.
They are inverses. Pace is time per distance, so lower is faster. Speed is distance per time, so higher is faster. Both describe the same performance from opposite directions.
Multiply a per-kilometre pace by about 1.609 for the per-mile equivalent, and divide going the other way. A 5:00 per kilometre pace is roughly 8:03 per mile.
Because pace and speed are inversely related. At a faster pace, the same time saving represents a larger increase in actual speed, so each successive improvement requires proportionally more.
Running the second half of a race faster than the first. It is widely recommended because starting conservatively preserves energy, and most personal bests are set with an even or slightly negative split rather than a fast start.