  Time lags due to X-ray reverberation have been detected in several Seyfert
galaxies. The different travel time between reflected and directly observed
rays naturally causes this type of lag, which depends directly on the
light-crossing timescale of the system and hence scales with the mass of the
central black hole. Featureless `hard lags' not associated with reverberation,
and often interpreted as propagating mass accretion rate fluctuations, dominate
the longer timescale variability. Here we fit our reltrans model simultaneously
to the time-averaged energy spectrum and the lag-energy spectra of the Seyfert
galaxy Mrk 335 over two timescales (Fourier frequency ranges). We model the
hard lags as fluctuations in the slope and strength of the illuminating
spectrum, and self-consistently account for the effects that these fluctuations
have on the reverberation lags. The resulting mass estimate is
$1.1^{+2.0}_{-0.7} \times 10^6~M_\odot$, which is significantly lower than the
mass measured with the optical reverberation mapping technique (14 - 26 million
$M_\odot$). When we add the correlated variability amplitudes to the time lags
by fitting the full complex cross-spectra, the model is unable to describe the
characteristic reverberation Fe K$\alpha$ line and cannot constrain the black
hole mass. This may be due to the assumption that the direct radiation is
emitted by a point-like source.
