On mechanism behind Marcus theory
The reorganisation is just the extension/contraction of of bonds and most importantly solvent motion /reorientation to equalise the energy at which point the electron tunnels and relaxation occurs into the product. Reorganisation happens before the reaction, it is, as it were, how the 'hill' is climbed to reach the transition state thats all. (This is the insight that Marcus had). Consequently it is the rate limiting process, even though it may take less than a picosecond in some reactions.  After reaction, relaxation (reorganisation) also occurs so that the solvent reaches equilibrium with the new charges distribution.

Figure source; R. Marcus, Biochimica et Biophysica Acta., 811, 1985, 265 - 322)