...if I go for a swim to feel better, and end up going slower than my usual speed, I feel like I've failed at something and my mood is worse, although physically I feel better.

I recently read through one of the Ericsson papers on expert performance (looking for sleep-related statistics for my melatonin article), and saw in http://projects.ict.usc.edu/itw/gel/EricssonDeliberatePracticePR93.pdf the following (emphasis added):

'In a longitudinal study of visual artists, Getzels and Csikszentmihalyi (1976) found that most artists were drawn to painting be- cause it allowed social isolation. However, aspiring painters have to promote social relations with art dealers, art critics, and buyers to gain notoriety, increase the demand for their art, and generate sufficient sales for full-time artistic activity. Failure to do so forced many of the best artists to take another job unrelated to painting. Once these artists could no longer commit sufficient time and energy to maintain and improve their performance they stopped painting completely because they could not accept performing at a lower level.'