A M-20 pilot conducting a practice ILS in IMC failed to execute the missed approach at the DH and continued down the runway at a dangerously low altitude without ground contact.

Narrative

I was conducting a proficiency flight in low IFR conditions. On and ILS approach straight-in; I requested and received clearance for low approach. After reaching an altitude near the DH; with the runway environment not in sight; I continued ahead for a not insignificant distance before executing the missed approach. The localizer needle was not centered (a fact the Tower Controller noticed on radar); indicating that I was probably not still over the runway upon commencing the missed approach climb. It was only after the completion of the flight that I fully appreciated the seriousness of the blunder. The published elevation of the touchdown zone is 173 FT MSL. The DH is 373. I stopped the approach descent at 400 FT on the altimeter; rounding up the DH in the direction of safety. A look at charted obstacles shows structures on the airport property up to 345 FT MSL (the Control Tower). Not much margin in instrument meteorological conditions. In retrospect; I believe the lowest safe altitude in the scenario would have been 600 FT MSL; the MDA for a straight-in localizer approach. (Or the circling MDA: 680.) So why hadn't I stayed at least that high; or why hadn't I executed the 'missed' upon reaching the DH with the runway not in sight? In instrument training; years earlier; I remember being taught to go 'missed' right away in that situation. We probably asked for 'the option' (which includes the option for full-stop landing) on those training flights. In more recent times; on practice instrument flights; I have gotten accustomed to requesting 'low approach' rather than 'the option' when I have no intention of landing. Then; after breaking out; I have gotten accustomed to flying visually over the runway before executing the 'missed.' I think it was that habit that created the mindset that led me to delay the 'missed' in this instance. It had been a long time since I had flown an actual ILS approach without breaking out. Evidently; I forgot what to do. Searching for a moral to this story; I took a look at the 'missed approach' definition: 'A maneuver conducted by a pilot when an instrument approach cannot be completed to a landing.' Well; if the instrument approach cannot be completed to a landing; it cannot be completed to a low approach over the runway either. You have to go 'missed' without delay. I'll remember that next time.