My urine is dark brown and murky after today 15k run. How long before worrying?

Get evaluated. Brown discoloration of the urine is usually attributable to blood, and in certain cases can be provoked by running or intense exercise. This is usually benign. A more serious condition is rhabomyolysis, where sustained, strenuous exercise can cause muscle necrosis, whose breakdown products appear in the urine. This is a serious condition that could lead to kidney damage and needs evaluation.
1-2 days. If your urine is dark and looks bloody, you likely have rhabdomyolysis (break down of muscle). The main concern is renal failure as myoglobin can harm the kidneys. Make sure to drink plenty of fluids. If you have very little urine or stop urinating, seek immediate attention at the emergency department. If your urine does not return to normal in a few days, see your doctor.
Get it checked out. Tea-colored urine usually means old blood. Excessive physical activity can trigger bleeding, but not usually in the absence of some underlying problem, e.g., bladder stones. It is worthwhile seeing a urologist to get the situation checked.