I am asking the question that the OP here started to but doesn't quite at the end.
Janet Van Dyne was lost in the Quantum Realm for 30 years. In those years, she gained certain abilities. These abilities, however unclear at the moment, do indicate that she can absorb and release quantum energy. Absorbing such energy could probably have kept her alive and fulfilled her body's energy requirements over the years. But she couldn't have learned to do so on the first day or week itself.
Some of the comments got me thinking and I developed a theory of sorts. When Hank and Janet embarked on missions, they probably carried food supplies shrunk with them. This would have helped on extended missions. This is however, purely a hypothesis and admittedly there is no in-movie reference to something like this.