Close - I have a marathon PB of 3:48 and on that day I passed through the half at 1:47 and 25km at 2:09, which was also a PB. I think it works better for longer distances - I've done sub 4 mins for 1 km, 23 mins for 5km but never dipped under 48 for 10km. I can run fast for short distances but have a great deal more strength and stamina than speed so do rather better on longer courses. As others have said, the other variables such as training, running history, weather, health, play a big part.