Workarounds for playing a game installed from a retail disk without an internet connection to download updates?
Solution 1:
Assuming your computer does have Internet access, but your monthly data limit is such that you don't want to use it to download updates to your computer then there might be a possible workaround to your problem. If there's another computer that you have access to that you can install Steam and where it's Internet access isn't as limited, say a friend or relative's computer, you can download the updated version of the game to that computer and then copy it to your own computer.
To do all this is fairly easy. Just download the Steam client from the Steam store on to the other computer, log in to your account and install the game on that computer. Once the game is downloaded you can backup the game to a USB stick or portable hard drive. It will need sufficient space, so you might need a 16 GB or bigger drive. To perform the backup right-click on the game in your library and select "Backup Game Files...". The game should already be selected in the list of the games to backup so press "NEXT". It will then ask you where to make the backup, so select the drive you brought with you. Once you done that and you've pressed "NEXT" you be asked how you want the files divided up. You can press just "NEXT" since you're not planning on burning them to a disk. The Steam client should then backup the game to your drive.
Now take your drive back to your own computer and start Steam. I'm not sure how well this will work if you already have the game installed, so uninstall it by right clicking on it and choosing "Delete Local Content...". After doing that select "Backup and Restore Games..." from the "Steam" menu at the top left. Select "Restore a previous backup" and press "NEXT". You'll then be prompted for the location of the backup, so use the browse button to select your drive. There should be a folder with the same of the game you backed up on the drive, so select that. The dialog should say "Program backups found: ", if not you need to find the subdirectory on the drive where the backups were placed. Then just press "NEXT" through all the prompts to install the game pretty much like normal. The Steam client will still need Internet access during this part, and might need to download a small bit of data, but not gigabytes.
The next thing you should do after you've launched the game and verified that it works is set the Steam client to Offline Mode. This is the "Go Offline..." option under the "Steam" menu. You'll want to keep the client in offline mode so it won't check for updates and you won't have to go through all this again the next time the game gets a multi-gigabyte update.
Solution 2:
Many retail games don't include all the necessary game data on the disk. In the case of Borderlands 2, it seems that it did not include the full game on disk, so an update is required to download the rest of it. Since a newer version exists since the disk was released, those remaining files turn out to be a majority of the game's file size.
Since it's not possible to originally obtain the data without using internet, here are several possible work-arounds depending on your circumstances.
If you're using a laptop, try bringing it somewhere with an internet connection. You don't have to download the full update all at once, and you can pause and resume when needed.
If you're not using a laptop, but you have one, try downloading it somewhere with an internet connection such as a school, library, or coffee shop.
Try to install Steam somewhere with an internet connection, download the game, and then use the backup and restore feature of Steam to transfer the files to a couple blank DVDs or a flash drive.
Use Family Sharing to give a friend access to play your copy of the game. In exchange, ask that they let you borrow a flash drive or set of DVDs with the backed up copy.
Download half the update this month and half next month so you don't go over your data cap.