Distance to the runway/ touchdown point
There is no distance to the touchdown point because there isn't a set touchdown point. there's no runway start distance because that's not what you are aiming for. The purpose of the ILS is to get you close enough to the runway to complete a landing visually. If the visibility is so bad you cannot see the runway even when you are right on top of it then you need an airplane with a completely automated landing system to legally shoot the approach.

What you do have a DIS (Distance) reading in nautical miles to the ILS at the top of the screen, which usually tells you all you need to know. This distance is what you use in the approach procedure which you get from the approach plate for the runway and category of approach you are doing. If you get to the missed approach point, which is a set distance from the ILS, without seeing the runway lights then you must go around. 

I'm going to assume you are working with a simulator like x-plane or MFS, in which case you aren't bound by procedure, and you can give it a try without endangering anyone. The ILS in most airports is placed right next to the runway, about where you'd want to aim a touchdown, so in a white out condition you would follow it down as far as you can until you contact the ground. It's been successfully done in emergencies, there's a good deal of pucker factor involved.