100K (~62 miles) is much easier than 100 miles. Many people have problems and "hit a wall" around 70 miles. In my experience pretty much anyone in reasonable shape can do 100K as long as they pace themselves. If you can do 35K and not feel like you are going to die at the end then you should be fine.
Following up on this,
From a site that discusses ultra-marathons (200 miles+), the best way to train for this is to try and do 2/3 of the distance up to two weeks or so before the event. Make sure that you can do this distance
safely. If you can complete that, then you will be able to complete the rest (though you might hit a wall because you're not used to that distance, like I did in my first century).
It's slightly challenging at first (depending on the route; if it's hilly or has a lot of rolling terrain, it will be more challenging), but it gets easier as you do them more often.
Oh, and:
Originally Posted by
ducksa
EDIT: They are allocating approximately 5 hours for the race, which means I have to average over 20KM/H including pit stops.
It's not a race, man. Please don't get into that mentality (though it's easier to think so at first because it sounds cooler. I did it!). It's a
ride.