As you found out Garmin's stock set-up is on Auto where, to save memory, it only logs a trackpoints when you change direction. So you did the right thing to change the recording to 2 sec.
Don't worry about the 10sec recording... the post analysising software can find your best 10sec out of the 2sec data.
Set at "garmin" (forget NMEA, that is for real time logging) and use G7toWin
www.gpsinformation.org/ronh/g7towin.zipto download data from the 72 to the PC/laptop and save it as a gpx file. Once it is in that form you can use GPSActionreplay (free) or Realspeed or GPSResults to analysis the data.
It's probably a bit late now but Decrepit is right.
The Navi/GT11 is a cheaper unit which is way more sophisticated. It can show in realtime your best top speed and reset itself automatically when you accelerate again, stores months of data at more accurate levels and can charge itself from your laptop or in the car. Tho' it isn't a mapping gps.