Paul's Law can help you identify if you need to work on speed or endurance. It is a rule saying that for a rowing athlete with balanced speed and endurance capabilities, every doubling of the distance should result in a five-second increase of the 500m split.
All you need to calculate the "predictions" of Paul's Law is a known time or speed for any distance. The formula behind this calculation is:
p₂ = p₁ + 5 log₂(d₂/d₁)
p is the pace in seconds, d is the distance in meters.
Paul's Law is quite controversial. Many people like it, and you can find lots of discussion about it in forums or on reddit. Some people hold against it that it is not very accurate, especially for long distances.