You just need to actually rotate the elevational views 90 degrees from the x axis and use the original y as the z axis and use that to pick the height. Another way to do this is to use "QuckCalc" ('quickcalc or CTRL+8). Begin the extrude command and when asked for a distance, run quickcalc and press the "Distance between 2 points" button. Then pick the 2 points you wish. The calculator will come back and then you just press "Paste to Command Line", hit enter, and it will extrude the object to the needed distance.