“I ran into someone I haven't seen in years.”
Earlier in the day, you saw someone that you used to know but haven't seen for several years & were surprised. You say this when you tell your sister about it later.
I ran into someone I haven't seen in years.
Talk about seeing someone for the first time in a long time with the phrase "I haven't seen ___ in ___":
I haven't seen you in over a year!
You use "in years" when you can't remember how long it's been since you saw them, but you know it was more than one or two years.
This means to unexpectedly meet someone you know or know about. For example:
I ran into a few former coworkers there. That was a nice surprise.
You'll never believe who I ran into at the supermarket: Leonardo DiCaprio!
A common place to "run into" someone is "on the street":
I can't believe I just ran into you on the street like that. What a coincidence!
That means outside in a public place, like on a sidewalk.