The dif is that unlike light years, 'many miles' is not a quotient of years.
It doesn't matter what it is a quoteient of. I can ride a bicycle a certain distance in a week. We can call that distance bike-weeks.
Would it make sense to talk about my age in bike-weeks?
No, because it is a measure of distance not time.
Light-years in reference to time is not different than just "years", or snail-years (the distance a snail can travel in a year).
Your phase would mean the same thing if you put "eons of snail-years". Why even put
light-years in there if you could have just said "years"?
How does including the distance that light can travel in that time help your point in any way? Your still just talking about the time portion of the phrase, and not talking anything about the distance that it is describing. Why is that?
I think its because you're trying to sound smarter than you actually are.