Hello hellen
The 100km from Biel was my first ultra i ever did. Its a very good beginner 100km because its not so steep and to run through the night was acctualy a big plus (heat, "mystical" night)
Another big benefit is, that it's only a 1x100km loop (most 100km runs have a 2x50km loop or similar). So you run never twice the same path. Not only the run, also the old towns you pass along the way help you a lot to get motivated all night
i did so fare around 10 Ultramarathons. Beside a run in Bangkok, it was the best ultra i ever did!
on the runningmonkey.co.uk page, you might find some more information and a race report
Hope i could help you and if you need more information im happy to help you (if you understand my crappy english
)
happy running