How do you train for a 100km race?

I had this question last night at track, and it got me thinking.  The answer I gave was short because my training plan was actually pretty simple.  Once a month, in the lead-up to the race, sign up for an ultra and run it as a training run.  Don't race it, but use it as an opportunity to get in a long run.  By doing this you convince yourself that you will be able to go the distance on race day.  Of course, practice your nutrition and pacing for your given goal (100km) race. 

With those ultras under your belt as a physical and mental base, you don't need to do a lot of 20+ weekly training runs. I find that those, if done too frequently, tend to just grind you down.  Instead, focus on weekly time on feet and mileage.  I count gym time as time on feet and, honestly, in addition to running practice ultras, the other big piece is doing strength training. I do hard/easy alternating weeks in the gym, with up to 5 strength sessions in a hard week.  On the easier gym weeks, you can ramp up the running mileage to make up for the time on feet short-fall, aiming to get 8-10 hours of training in per week.

Aside from the above (ultras as training runs and gym), you just go about your business as normal: do the Monday night runs and track with the club, and lead Sunday runs in the forest, and then run three more times per week totalling six times per week for an average mileage between 40 and 60 miles per week.  My highest mileage week was 62, and I didn't run more than 20km in one go that week.  My longest run overall through the whole training block was a 33 mile run across Dartmoor, in harder than race day conditions.  

Completing my first 100km non-stop took a long time to work up to. I would say it took me 5 years to finally reach my goal: from my first ultra in 2019 when I did part 2 of Race to the Stones (5h 30mins coming in 10th place overall), through my unsuccessful attempt last year in 2024, until this year's concerted effort to get organised and mentally optimistic with the explicit goal of completion.  So, it does take some time, and it's by no means easy.  But if you train right and are smart, you can make it look relatively easy on race day!