Trekking is an interesting word, referring to the making of a slow or arduous journey. In the sense of getting out & about on a bicycle, the word is contrasted with bicycle touring, which seems to refer to sight-seeing on a bike for pleasure and recreation.
Can bicycle touring be slow and arduous? Most definitely! But it doesn't have to be.
You can bicycle tour without trekking, and you can trek on a bicycle without touring. But in the sense of long-distance recreational bicycling, there's a certain room for overlap in usage.
So, why am I Trekking Bob, and not Touring Bob? Well, I think of touring as something I'd ultimately like to do, but as a rule I'm more into getting out and doing the challenging stuff than I am into sight-seeing. I also like to swim up near my limits and do other things that are a real stretch.
According to dictionary.com...
"Trek was borrowed into English in South Africa, where the word was used by the Boers for a journey by ox wagon. A seminal event in the history of South Africa was the “Groot Trek” from 1835-1843, in which more than 10,000 Boers, the Voortrekkers, left the Cape Colony and traveled north and northeast because of economic problems, conflict with the Xhosa, and discontent with British colonial authorities, who had forbidden the slave trade and postulated the equality of whites and non-whites. The British, who seized control of South Africa from the Boers at the turn of the 20th century, seized the word trek during the 19th. Trek is recorded earliest in 1822 in the compound trektow, “a rope joining the wagon pole and the yoke to which oxen were fastened.” Trek in this compound is either the noun or the stem of the corresponding verb in Afrikaans, trekken. The earliest recorded use of the noun by itself is found in 1849, where it means “a stage in a journey by ox wagon."