Vocabulary question

[Melody]

but again what´s 100k or 200k compared to 100 mile/km ride?

Laura, if you need to do a metric to imperial conversion you can get google to do them. Go to http://www.google.com and in the search field type

100 kilometer to miles

and google will give you an answer. You can do this for most measurements. I use it quite a bit when I'm cooking and having to change portion sizes.

So now I can tell that I'm going to do a 3/4-century (70 km) [...]

You can also just say "I'm doing a 70k". Also, when talking centuries, if you're in the US, typically they're 100 miles and to differentiate between 100 kilometers you'll here the term "metric century" instead.

Oh btw peloton is a Finninsh word as well! I got a bit confused since here it means fearless.

Believe it or not everyone, peloton is also an English word. From the Oxford English Dictionary

I. Simple uses.
b. Cycling. A group or cluster of cyclists in a cycle race; esp. the main body of competitors, the pack (PACK n.1 7d)

The etymology of the word, however, is French

[< French peloton little ball, esp. of thread (1417 in Middle French), small body of soldiers (1616), group of competitors in cycling (1884; 1855 in a sporting event in general) < pelote (see PELLET n.1) + -on (see -OON). With sense 1a cf. earlier PLATOON n. and also Spanish pelotón (1737 or earlier in this sense).
N.E.D. (1904) indicates the stress as pelo{sm}ton.]

It looks like the very base root of the word comes from a small body of soldiers. Someone probably, at some point, said that the group looks like a peloton of soldiers and the saying stuck.

Mel the mad OED user