…or is it ‘fast packing’, or ‘speed climbing’, or what?
It all depends, on the trail, and on who does it. We recently published a short piece of fast packing, and argued that this shares a lot with trail running. Cover more trail ground than normal in a particular amount of time. Fast packing is nearly by definition multiple day, and thus compares best with multi-day runs like the Annapurna Mandala Trail, the Himal race, or the Solukhumbu Trail.
Where does running start and walking end? Most up-hills are walked on the multiday trail runs, some elite runners may run part of them but your average participant won’t. A Fast packer wouldn’t run the flats but might do the down hills nearly as fast as a runner, depending on her technique. Ever come across a Nepalese porter in a hurry on a down-hill? Running, walking, whatever they do, you have to be a technically pretty skilled down-hill runner to get anywhere close to their speed.
Now the other end of the spectrum: When Kilian Jornet breaks the record for running up and down Kilimanjaro no one would doubt this as a superb feat of trail running. Those into fine distinctions might argue that this should be called mountain running, those into finer distinctions would argue for sky running, both kinds of pundits would say it is certainly not fell running, but none would dispute it as a running performance. Recently I came across Ueli Steck, a Swiss speed climber, and watching this video wondered about the similarities and differences. This was certainly about covering more ground than normal in a particular amount of time, the ground might be different, but the concept? The different ground makes for some additional requirements, like gear that no normal “runner” would ever carry around but does that really matter?
On to pursue another line of comparison: when I run trekking trails that I am not yet familiar with, I carry a map. If I would want to make it more extreme I might leave the trails and bushwack my way to where I want to go, and suddenly similarities with fell running would increase, and others might say that I am now orienteering.
So there is a whole bunch of outdoor pursuits with family resemblances, one blending into the other. For completion purposes they are pigeon-holed into strict definitions, but out there, on the trails, they all share more than they differ.