I don't know if you already thought of this, but your question is related to a result of Balacheff and Sabourau (an electronic version of their paper is in Stéphane's [web page][1]). Without going into the technical definition of the diastole over $1$-cycles, their result basically says that *there exists a constant $C > 0$ (which, unfortunately, is very small) so that given a Riemannian metric on the sphere with area $4\pi$, there exists a Morse function such that the length of any of its level curves is less than $C$.* This is the best that is known in this direction.

On the other hand, a related conjecture which has at least a shot at being true runs as follows:

**Conjecture.** *If the area of a Riemannian two-sphere is $4\pi$, there exists a closed geodesic that is regular homotopic to a figure 8 and whose length is less than or equal to $4\pi$.* 

Note that twice an equator in a round sphere is regular homotopic to a figure 8. 

In fact, the conjecture should be for all (reversible) and non-reversible Finsler metrics and equality should hold only for Zoll metrics. The reason is that this conjecture is a consequence of (a reasonable extension of) the Viterbo conjecture.
Enough is known about this conjecture (Artstein-Avidan, Ostrover, Milman, Alvarez Paiva and Balacheff: see the **first version** of [Contact geometry and isosystolic inequalities][2] and the references therein) that an optimist may harbor some hope of its validity.


  [1]: http://perso-math.univ-mlv.fr/users/sabourau.stephane/
  [2]: http://front.math.ucdavis.edu/1109.4253