Training intensity distribution (TID) in recreational runners. The TID is reported as training completed in each zone in males and females (panels a and b, respectively), and younger and older marathon runners (panels c and d, respectively). Each panel shows the total training time completed in zone 1 (green), zone 2 (yellow) and zone 3 (red). Runners were grouped on the basis of their marathon finish time in 30-min bins (120–150 min, 150–180 min, etc.), with the average marathon time and the number of runners also displayed