In this paper we explore the dynamics of the mixed Stark manifold of ultrahigh Rydberg states (principal quantum number n = 50-250) of the large molecules interrogated by time-resolved ZEKE (zero-electron kinetic energy) spectroscopy. We pursue the formal analogy between the coupling, accessibility, and decay of ultrahigh Rydbergs in an external weak (F = 0.01-1.0 V/cm) electric field and intramolecular (interstate and intrastate) relaxation in a bound level structure. The effective Hamiltonian formalism with several doorway and escape states was advanced to treat the dynamics. The theory accounts for thr dilution effect, i.e., the dramatic lengthening of the lifetimes of ultrahigh Rydbergs, relative to that expected on the basis of the n(-3) scaling law for the decay widths. Model calculations for the field-induced (I) mixing reveal that die Rydberg time-resolved population probability is characterized by two distinct (similar to ns and similar to mu s) time scales. To date, long time-resolved (10 mu s-100 ns time scales) nonexponential decay of ZEKE Rydbergs was experimentally documented, in accord with our analysis. The predicted existence of the short decay times (1-10 ns) was not yet subjected to an experimental test. Next, we extend the model calculations to treat the mixing of several n manifolds, demonstrating that in the strong mixing limit the overall features of the temporal decay are similar to that of a single n manifold.