Things to Note

1. Book flights at least four months in advance for cheaper fares.
2. For bringing your bike box, both China Airlines and EVA Air offer 30kg of free baggage, but China Airlines currently only supports hard-shell cases. Notify the airline in advance about the oversized luggage.
3. Assembling the bike takes time, so practice at home — otherwise you'll spend all your time on transport, assembly, and disassembly.
4. May and June are the rainy season, and weather around Lake Kawaguchi changes drastically. We didn't get to see Mt. Fuji this trip. "The race has been held in the rain two years in a row now."
5. When removing clipless pedals, remember the right pedal is reverse-threaded and the left pedal is normal-threaded.
6. From Shinjuku to Lake Kawaguchi, you can take the Fujikyu Bus to save some money — JP 1500 one way, with a huge lower-deck luggage compartment.
7. Lodging at Lake Kawaguchi is pricey and gets booked up fast — reserve early.

Race Day Tips

1. The official documents are only in Japanese. Many things are written but not clearly, and we missed the 6:00 baggage drop-off, so we froze at the top. (At the summit with heavy rain, it dropped to 7°C.)
2. One unique thing about Japanese races: there may be no aid stations along the course — only water and restrooms. Bring your own supplies. There's a shop around the 20km mark where you can buy food. I've heard the lack of aid stations is for environmental protection, but most riders bring their own food anyway, and there's still plenty of trash along the road. It would be better to consolidate that at aid stations. For first-timers like us, the lack of information was genuinely frustrating — both my friend and I felt this was a really weak part of the experience!
3. If you missed the baggage drop, head to the stadium at the northern foot of Mt. Fuji — there are 50-yen lockers next to the restrooms.
4. With cold weather and high energy expenditure, your hands and feet go numb. Going to the bathroom is awkward, and lactic acid buildup makes recovery harder.
5. Be careful on the descent. I saw two cyclists collide. Japanese riders tend to stick to the far left and the middle, but you can actually ride a bit left of center to reduce collision risk.
6. The official Reric race jerseys run small. They use a Lycra-like blend, so refer to Shimano Asia sizing and go up one size.
7. Things around Lake Kawaguchi are pricey, and the cheap options aren't filling or to our taste. To save money, eat at convenience stores and supermarkets, and have a proper meal back in Tokyo.

FAQ

Would I come back to Mt. Fuji?
Definitely, but probably not during the rainy season. I felt the organizers' rain contingency and on-site support for foreign participants were lacking — staff weren't very clear, "documents had no English version", and race-day commitments tied up all our time. The wind and rain made it a memorable experience, but aside from the race itself, the trip felt incomplete.
Finally, thanks to my coworker and friends for their Japanese language support and travel intel.
Costs (will update after returning to Taiwan)
Taipei–Tokyo round-trip flight: NT 11,500
Narita Express (Shinjuku–Narita): JP 3,200 x 2
Fujikyu Bus Shinjuku–Lake Kawaguchi round trip: JP 1,500 x 2
Lake Kawaguchi lodging is more expensive, so my coworker and I split it
Shinjuku Central Hotel: NT 3,800 / 2 people
Airbnb for 2 nights: NT 6,400 / 2 people
Mt. Fuji Hill Climb registration fee: JP 1xxxx
Akihabara Remm Hotel: NT 3,200
Overseas travel insurance, 5 days 4 nights: NT 7xx

Photos