Pace, time and distance
Pace is just time divided by distance, and the three quantities lock together: give any two and the third is fixed. Runners in the US usually work in minutes and seconds per mile, while race splits and most of the rest of the world use minutes per kilometer. One mile is 1,609.344 meters, so a mile pace is always about 1.61 times the kilometer pace. Speed in miles per hour is the same information upside down: 3,600 divided by the pace in seconds per mile.
A worked example: 5 kilometers in 25 minutes flat. Five kilometers is 3.107 miles, so the mile pace is 1,500 seconds / 3.107 = 482.8 seconds, which is 8:03 per mile. In metric it is simply 25 minutes / 5 km = 5:00 per kilometer. Average speed is 3.107 miles / (25/60) hours = 7.46 mph, or 12.0 km/h. Turn it around and a runner holding 8:03 per mile for a marathon (26.219 miles) would finish in 3 hours 31 minutes.
Predicting other distances with Riegel's formula
Pete Riegel published his endurance model in 1977: T2 = T1 x (D2 / D1)1.06. The exponent above 1 encodes the fact that you cannot hold short-race pace over a long race; each doubling of distance costs roughly 6% more than proportional time. From the 25-minute 5K, the model predicts about 7:31 for a mile, 52:07 for a 10K, 1:55:00 for a half marathon and 3:59:47 for a marathon.
The formula works best between about 5K and the half marathon, and for runners whose training actually supports the target distance. Predicting a marathon from a 5K almost always flatters the runner, because the marathon is limited by fuel, heat and muscular damage as much as by aerobic fitness. Many coaches raise the exponent to 1.07 or 1.08 for marathon predictions from short races, which adds several minutes. Treat the marathon row as a ceiling you earn with long runs, not a promise.
Using the split table
The split table shows the clock time you should see at each mile or kilometer marker if you run perfectly even splits, plus the leftover fraction at the end. Racing to a split table is the simplest form of pacing discipline: check the watch at each marker, and correct gently rather than in one burst.
Even pacing is close to optimal in theory, but in practice a slightly negative split, running the second half a little faster than the first, produces the best results for most people. Go out 5 to 10 seconds per mile slower than goal pace for the first mile or two, settle in, then spend what is left. Adjust expectations for conditions as well: heat above roughly 60 F, humidity, wind and hills all cost time, and a rule of thumb is about 20 to 30 seconds per mile slower for every 10 degrees above 60. Hilly courses cost more going up than they give back coming down.