It may help if I suggest two ways of looking at the situation.
- The first way, which tries to reflect your line of thinking (as I understand it), looks at the situation as two reactions being needed to maintain ADP in the cell. The first reaction produces ATP:
Glycolysis: ADP + Pi → ATP
Whereas the second reaction regenerates ADP by using the ATP
Biosynthesis / Movement / Electrochemical work etc: ATP → ADP + Pi
If the second reaction doesn’t occur, the first one will stop too, but that is OK as the cell has no need to produce ATP if there is plenty.
- A second (perhaps more sophisticated) way is to regard the situation as an equilibrium:
ADP + Pi ⇋ ATP
The position of this equilibrium (and hence the relative concentrations of ADP and ATP) will vary depending on the situation in the cell. The concentrations of ADP and ATP will effect glycolysis (and other aspects of metabolism) both by simple mass action, and also be something you may not have yet met, the regulation of certain key enzymes (allosteric regulation). (An equilibrium between ATP, ADP and AMP also exists.)
Finally, there are both open systems and (semi-) closed systems in biological chemistry. The conversion of foodstuffs into chemical energy and the structural components of a cell is obviously an open system — the substrates for glycolysis ultimately come from outside the organism. The interconversion of ATP and ADP (and NAD+/NADH) can be regarded as (semi-) closed, as the main part of the chemical moiety is being conserved. However one should not forget that, initially, when the organism is growing and cells are dividing, the adenosine backbone has to be synthesized, and it requires energy to do this and more energy to add phosphates (also from the diet) to the ribose.