A few years ago, researchers in Taiwan recruited volunteers for what they were told was health-related product testing. Their first task was to take a pill and rate its texture and colour; half were told it was a multivitamin, while the other half were told it was a placebo (which it was). During the next tests, the groups acted dramatically differently. When asked to test a pedometer, those who thought they’d taken a vitamin were more likely to walk a shorter route; at lunch, they chose less healthy food. In a survey, they expressed greater feelings of invulnerability and less desire to exercise.

What does this have to do with running?

Racing your best demands that you co-ordinate countless details of training and lifestyle – long runs, intervals, sleep, nutrition, and so on – that contribute to your performance. This study illustrates a phenomenon known as the ‘licensing effect’: doing something you believe will help you (like taking a vitamin) will subconsciously encourage you to slack off on other ways of achieving the same goal (like eating well).

You must distinguish between the factors that matter for your fitness, and the ones you can ignore – because you’re always making trade-offs. Here are three to watch out for: