In this sentence alone, it's not clear whether 'longest' means time or distance. Do you have the whole paragraph it is in?
I think it refers to distance, as it really goes through 11 cities.
The term "longest" refers to the greatest distance or length in this context. So when we say that the Eleven Cities Tour is the longest speed skating race, it means that it covers the greatest distance compared to other speed skating races. In this case, the "longest" refers to the length of the race, not the duration or time it takes to complete it.
