The following chart shows a comparison of the average daily mileage the fastest runners and hikers manage to complete on selected long-distance trails. As one can see, the shorter the trail, the more miles per day are possible. That intuitively makes sense. The color represents the trail while the bubble size represents the “style,” i.e., supported, self-supported, or unsupported. Most longer trails can obviously not be completed unsupported. A notable exception is Ultrapedestrian Ras’ unsupported Washington PCT FKT (541mi). Unsupported tends to be the slowest, while supported and self-supported are generally faster.
The 750mi Oregon Desert Trail (ODT) is among the slowest trails, partly because there are 267mi (36%) of cross-country travel to survive, many of them in very challenging terrain. See ODT trail surface plots for more details.
The following raw data was used for the chart above. A GoogleDocs spreadsheet is available here.