In short its fine you have changed who can access drive(s) that are mounted there.
/run/media/username/* is used for automatically mounting external devices. I usually manually set mount points for reguarly used drives to /mnt/* and set permissions on a per drive basis but its not the end of the world you've basically given all user accounts on your system read, execute permissions to where external drives are automatically mounted for your user. (From 750 to 755)
Setting 755 on a folder means it has the following permissions:
- Owner has: read, write, execute
- Users who are a member of the group the folder belongs to have: read, execute
- Other users have: read, execute
The owner having full permissions makes total sense. While you can of course set tighter restrictions, for a folder to be browseable / able to be entered, it needs at least read and execute permissions or you will get errors.
So as it stands now everyone who has an account on your system is able to access the contents of /run/media/preston/ if this is undesirable, set the directory back to 750 instead and setup /mnt mount points for regularly used drives where you want more control over permissions.
EDIT:
My system's setup as example
stat -c "%a %n" /mnt/* <-- gets permissions in octal
700 /mnt/DataDrive
755 /mnt/Movies
755 /mnt/TV1
755 /mnt/TV2
