How to calculate running pace
Pace = time ÷ distance. If you run 5 miles in 40 minutes, your pace is 40 ÷ 5 = 8:00 per mile. For a 5K (3.107 miles) in 30:00, the pace is 1,800 s ÷ 3.107 = 579 s, or 9:39 per mile, which is 6:00 per kilometer. To find a finish time, multiply pace by distance; to find distance, divide time by pace.
Converting pace between miles and kilometers
One mile is 1.609344 km. Multiply a per-kilometer pace by 1.609344 to get the per-mile pace, and divide to go the other way: 5:00 per km is 8:03 per mile, and 8:00 per mile is 4:58 per km. Speed is the inverse of pace: 60 ÷ pace in minutes gives mph, so 8:00 per mile is 7.5 mph and 10:00 per mile is 6 mph.
Race finish times by pace
At 8:00 per mile: 5K 24:51, 10K 49:43, half marathon 1:44:53, marathon 3:29:45. At 9:00 per mile: 5K 27:58, 10K 55:55, half marathon 1:57:59, marathon 3:55:58. At 10:00 per mile: 5K 31:04, 10K 1:02:08, half marathon 2:11:06, marathon 4:22:11. Race distances: 5K = 3.107 mi, 10K = 6.214 mi, half marathon = 13.109 mi, marathon = 26.219 mi.
Reading the race estimates
The estimates assume you hold the same pace for the whole distance. In practice most runners slow down over longer races, so a pace you can hold for 5K is usually too fast for a marathon. Use the table as a target for even pacing, not as a prediction of what you can run.