I do the park run every saturday (71 so far) and i always get a headache after which mostly lasts the whole day, by sunday i am fine.

See below : Primary exertion- or exercise-induced headaches: uncommon headache type; bothersome; harmless. Exact cause unknown: possibly related to blood vessel dilation inside skull. Usually resolve after 3-6 months. Rx: avoid dehydration; beta-blockers; NSAIDs taken 30-60 min before run helpful. Important for MD to first rule out potentially serious secondary causes such as heart or intracranial disease.