Put your iPad back into recovery mode.
Try to restore again and accept the T&C for iOS 11, then this time when you get those greyed out buttons, look above in iTunes on the upper right hand side and look for a box with a download arrow with a moving circle around it.
Click on that box and it will tell you how long the update will take to download.
The recovery screen is meant to timeout every 15 mins due to it not being directly accessible by the general public. Also if someone accidentally resets their iPad and the recovery screen pops up, that time will get the tablet back on the lockscreen for the average user since they need to be on that side of the software.