Here’s a table that shows a list of paces ranging from very fast to rather slow, the equivalent speed, an estimate of the power required, and the corresponding time that running at that pace will achieve for 5km, 10km, half marathon (21.0975km) and marathon (42.195km) distances. A custom distance can also be specified.

The power estimate is based on the ACSM running equation applied to a flat course, which estimates oxygen consumption using an amount that increases linearly with speed plus a small fixed cost, and converted to watts assuming a metabolic efficiency of 21%. Metabolic efficiency varies from person to person, as does biomechanical efficiency (how efficiently you turn body movements into forward motion), so these power numbers are more of a suggestion than an absolute guide. Triathletes who have a bike power meter tend to find these numbers reasonable, although it should also be noted that running and cycling fitness don’t directly cross over, so it requires training in both sports to be able to perform the similarly in both. This model was designed for speeds higher than 8km/h or 5mph and it is expected that this will overestimate power for lower speeds.

(Despite its name, this site can be used for running too.)