It uses a 1:45 'thon as baseline, predicting that runner will come in at 3:54 (1:45/2:09). Yikes. I guess it takes into account fatigue. But it also says higher distance than 30-40 mpw may mitigate that.
But the Riegel formula falls apart when you move up to the marathon. A typical runner in our study with a 1:45 half-marathon time (8 minutes per mile) had a marathon time of 3:53:10 (that’s 8:54 per mile). Yet the Runner's World calculator predicts that this runner will finish in 3:38:55—15 minutes faster than the actual result. That’s an “absolutely massive” error, says Vickers. If this runner paced himself according to the Riegel prediction, he would start the race at a pace that was more than 30 seconds per mile too fast. “No wonder so many runners blow up and limp to the finish,” Vickers says.
Our formula predicts that a 1:45 half-marathoner who trains 30 to 40 miles per week will finish a marathon in 3:54:50. That’s still a little bit off from the real-world results we saw in our survey—a minute and 40 seconds slower than the actual time.