Reflecting on a week exploring all 4 Mt Fuji trails - they certainly have their own personalities.
Mt Fuji ascent #1 - Subashiri trail
pre-existing FKT Strava segment - https://www.strava.com/segments/12809882
Profile/terrain:
Subashiri trail ascent has an increasing gradient across rock/grass/dirt terrain, transitioning to loose dirt and finally more of a rock climb/navigation. While the majority of the climb comes in the form of switchbacks, just when you start to get really tired, you'll be hit with 3-4 straighter/steeper ascents on loose dirt - bring your poles. Those come right about the time this trail joins with the most popular Yoshida trail (from 8th station onward), so you'll be dodging an increasing number of people going both up and down the mountain near the top. Fortunately, some of the trail does separate those ascending versus descending, which is a brilliant godsend at this gradient.
The final ~300m of the climb is steep, awkward and can be very dense with people. Your finish line is a large Torii (gate) where many people stop for a quick picture. Please don't make a photo shoot out of it - everyone is struggling to keep moving their legs by that point and it is pure torture to stop and start again.
For those not acclimated to altitude, please note you are starting at ~2000m and finishing at ~3700m, which will hamper breathing/performance.
Planning/provisions:
You now need to make a reservation to ascend any of the 4 trails, for safety tracking and capacity reasons. In Subashiri's case, that reservation is free, and you can do it on the spot when you arrive at the 5th Station to climb. If you begin your climb after 4pm, you must also have made an overnight mountain hut reservation. You can catch the shuttle bus to the trailhead from Gotemba Station (tickets available at station and on bus).
Mt. Fuji has no running water. This includes all mountain huts! You can restock your supplies by purchasing drinks/noodles/curry from mountain huts, or from the plentiful vending machines found at the start, the peak, and most huts. There are also paid toilets along the way. Bring Y100 coins with you for toilets/machines.
Please note also that if you aren't climbing "in season" (early July to early September), most facilities incl. vending machines will not be available.
My experience on the day:
Weather was humid, hot, and rainy - typical of Japanese summers. Got a bit lucky to have just a few sprinkles. Even without the rain, it was an absolute sweatfest. i was also lucky that virtually everyone respected that uphill climbers have the right of way.
Post-FKT, in order to have the widest choice of shuttle buses off the mountain, i descended via the Yoshida trail, which is not technical/rocky (though does come with a mandatory Y2000 payment to use, even on the downhill).
For weather, terrain, and gradient reasons, plus the looooong transit generally required to even reach the trailheads, i found Mt. Fuji to be one of the most grueling ascents i've done.
I made the climb Unsupported, with 1.5L of liquids and 3 gels/gummies (about half of this ended up saved for the descent and long way back to my accommodations).