On good runs, I'll set my body to autopilot and daydream so intensely that I'm almost surprised to arrive home at the end. On bad runs, it's not so easy. When I can't slip into that relaxing, meditative head space, I try to just distract myself. I'll look at the scenery or watch people around me, think about something I'm looking forward to (a holiday, a meal, a show), focus on my breathing instead of how stiff my legs are. You could also listen to a podcast or an audiobook. A lot of what makes a run "bad" or "good" has to do with your mental state, and focusing on something other than stiffness or exhaustion really helps.
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