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:
There are ten beers in my fridge.
Ten beers is a lot to drink.
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 last fifty miles of that route are the most scenic! [Among the fifty miles, individually, there are found the best views and points of interest.]
Fifty miles is a long way. [The quantity fifty is a long way, if the units are miles.]
The is/are applies even if the noun is elided, and only the number is present.
How many beers do we have? Twelve are in the fridge, six more in storage.
One or two beers isn't a lot to drink. But ten is.
Fifty miles here is considered as the entire journey (all those miles collectively). Something like 24 years is not a small period.