I came across an article that contains a pair of seemingly useful suggestions:
1. Resurrect the old collapse-point theory, which guided marathon training programs of the 1970s and ’80s. This grimly named system held that you can run about three times your average daily distance before hitting a wall. Marathoners, then, need to up that average run to nearly nine miles to push the wall past the finish line.
2. Limit the length of long training runs to half-again the daily average. Adding 50 percent to an everyday eight- or nine-mile run would place the longest one at about a half-marathon.