Sort your library by Kind. (If "Kind" is not visible, enable it by going to View > View Options and checking the box.) Note the ones that are Apple Lossless or AIFF, and drag them to a playlist.
Set your import option to AAC with your preferred bitrate. Go to your new playlist. Highlight a batch of them, and use the Command Advanced > Create AAC version.
iTunes will do the conversions serially, which will take a while, but you don't have to sit there.