Depending on your definition of a ‘long time’, one strategy is to run the Arch archives (see Shift Time here on the site if you don’t know how it works - or Arch wiki) and update to intermediate times in steps until caught up. A nuisance - but likely to avoid massive problems!
Essentially you tell the system to update to a date in the past (using changes in the mirrrorlist, most easily) and pacman -Syyuu for your updates. You can be reasonably sure with 1 month steps, but larger my be OK too - your call.
Below is the post with my little script (and welcome add-on) for making it simpler 