Terminal and ‘do shell script’ have absolutely nothing to do with each other even though they both deal with shell commands. Do you actually want a Terminal window open or do you just need to run the script without further interaction?
Since you don’t need the terminal for this, you can:
do shell script “ifconfig en1 mtu 1454” with administrator privileges
By excluding the password parameter, the script will actually prompt you for your admin password before running the script. If you’re happy embedding your password in the script then feel free to add the password parameter.