The razor tool is only an option if you work in multitrack mode. This is non-destructive, and gives you the ability to split tracks and move them around. With voice-over work, one way of achieving what you want to do is to put cue marks in your original recorded file (or preferably a copy of it - working with originals isn't recommended, as mistakes can happen...), turn these into cue ranges, and then by right-clicking on the range in the marker list, you can export the range as a 'clip' directly into multitrack view, and then manipulate it as you want.
Since multitrack view is non-destrictive, the clips aren't really 'in' it at all - it's just replaying your selected portion at the point you've put it. This means that if it's too long or short, you can extend it or reduce it just by dragging the ends of the clip. And as such, it doesn't disturb your original recorded file (copy) in the slightest, so you can redo things to your heart's content until you are happy with the results.