Earlier in the day, you saw someone that you used to know but haven't seen for several years & were surprised. You want to tell your sister about it & say:
I ran into someone I haven't seen in years.
You can use this phrase when talking to anyone - it's not especially casual or formal. You use "in years" when you can't remember how long it's been since you saw them, but you know it was a long time.
This means to meet someone unexpectedly. For example:
I ran into a few former coworkers there. That was a nice surprise.
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.
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