Well, an ultrasonic sensor would work quite well for that.
The Maxbotix sensor you got in the KOP is good out to around 20 feet, or maybe a little more. Keep in mind that it has lag time: it works best if you're sitting still.
They are generally accurate to about an inch, and are fairly easy to wire. They attach to one of your analog inputs on the cRIO.
You do need a flat surface in front of you, but since the wall is flat this year you should be fine. Of course, if a robot drives in front of you it will return bad readings.
What many teams do is to use several buttons, each which fires at a different speed or height. Then you let the operator figure out which one to push.
Finally, you'll find excellent help with LabVIEW at frcmastery.com. They have a series of videos that are slightly outdated but very helpful. In addition, National Instruments has a wonderful series of videos and documents to help you learn. Most of the links can be found here (only because it's easier for me to remember our web page than all of NI's):
Team 1718 Links Page
Keep in mind that our FIRST controls documents are from 2012, and we intend to remove them since FIRST has asked that they be the only repository of their documentation.