Fifty miles is a long way to ride a motorbike.
Why use is with miles which looks like a plural noun? Can you explain me what the rule is?
Let us compare two key sentences:
The difference is regarding the beers as individual items, versus a single numeric quantity (where "beers" serves as the unit, just like meters, seconds or kilograms).
The is/are applies even if the noun is elided, and only the number is present.
Fifty miles here is considered as the entire journey (all those miles collectively). Something like 24 years is not a small period.