The week-end before last I ran the first leg of the Hyde Park Relays, as part of the team "Baton Tossers". I managed a 19.25 time on the 5.164km loop. This is not too bad given my current general lack of serious training, especially at high speed/short distance. This is the shortest race I've run since my first ever race more than 10 years ago. Using the Riegel prediction formula, that would give me a marathon time of ... 2:59:58! I guess I can keep training.

Not so ultra.

PS: quite a few people, like here, say that the formula means that the "speed declines by around 6% when the distance doubles". I cannot really agree with that. Consider Riegel's formula t2=t1*(d2/d1)^1.06, and assuming d2=2*d1, then t2=t1*2^1.06. The speed v2=d2/t2=(2*d1)/(t1*2^1.06)=v1*2^-0.06=~v1*0.96. That is a reduction of speed of 4%, not 6%. Am I missing something here?