Just to put a little more flesh on the bones of what IO540 has said, the TDME is usualy paired with an approach aid like an ILS, the reading is usualy set to zero at the touch down point of the runway.
Most of the single runway airports in the UK have ILS on both ends but only one frequency so they switch off the ILS on the end that they are not using, the TDME is located at the runway mid point and is adjusted so it will read zero at the touchdown point at both ends (if you are nearer to the DME than the TDZ then you get an error flag).
There are moves afoot to stop this practice because it feeds errors into the FMS computor of airliners, most modern airliners use DME/DME as the primary navigation sorce, for this they are fitted with 5 DME units three of them permanently connected to the FMS system, this DME/DME fixing is more accurate than GPS untill they get hold of a TDME that has its own error (of half the runway length built in).
For your VFR navigation using the TDME is a very good idea for backing up DR & map reading, at the alitudes that you are flying DME range should not be a problem unless it says Narco on front of the DME box.