ABSTRACT Long-distance run training, a prolonged, cognitively undemanding activity that usually takes place in a familiar setting, creates conditions conducive of mind-wandering, i.e., thinking about something else than the current task or environment. The aim of the study was to explore some possible determinants and consequences of mind-wandering during the runner’s training. Fifty three amateur runners aged 18–54 took a 40-minute outdoor run at a comfortable pace. Immediately before and just after the run, they completed UMACL, a three-dimensional mood questionnaire. After the post-run measurement of mood, participants filled out a retrospective questionnaire concerning thoughts they had had during the training. In a separate session, working memory capacity (WMC) and individual tendencies in mind-wandering of participants were assessed. Participants’ mood significantly improved between the pre-run and post-run assessment on all three dimensions captured by UMACL: hedonic tone was more positive, energetic arousal was larger, and tense arousal was smaller after the run than before it. This positive shift in mood was more pronounced in runners who declared more frequent thinking about the future during the run. The frequency of such thoughts was positively associated with the runners’ WMC and the propensity to engage in positive and prospective mind-wandering. Also, WMC correlated positively with mind-wandering in general, negatively with the frequency of thoughts about aversive aspects of the training, and positively with the frequency of thoughts about pleasant experiences associated with the run. In all, these findings suggest that mind-wandering and WMC may be important factors in the psychology of long-distance running.