How fast should you swim in training?
It’s a simple question, but there isn’t a simple answer as it depends on a range of factors including your fitness, training goals (both long term and in each individual session), training history and the general stresses and strains in your daily life. But there is a way to start thinking about it, and that’s the concept of critical swim speed or CSS, which is the subject of a surprising amount of scientific research.
CSS is defined as the theoretical maximal swimming speed that could be maintained without exhaustion for a long period of time. You can therefore immediately see the relevance to open water swimming where the majority of events are long distance. If you can improve your CSS, you will be able to race faster in open water. But first you have to identify what yours is.