We present extensive follow-up time-series photometry of WD J0049‑2525, the most massive pulsating white dwarf currently known, with Teff = 13,020 K and logg=9.34 cm s‑2. The discovery observations detected only two significant pulsation modes. Here, we report the detection of 13 significant pulsation modes ranging from 170 to 258 s based on 11 nights of observations with the New Technology Telescope, Gemini, and Apache Point Observatory telescopes. We use these 13 modes to perform asteroseismology and find that the best-fitting models (under the assumption of an ONe core composition) have M⋆ ≈ 1.29M⊙, a surface hydrogen layer mass of log(MH/M⋆)≲‑7.5 , and a crystallized core fraction of >99%. An analysis of the period spacing also strongly suggests a very high mass. The asteroseismic distance derived is in good agreement with the distance provided by Gaia. We also find tentative evidence of a rotation period of 0.3 or 0.67 days. This analysis provides the first look at the interior of a ∼1.3 M⊙ white dwarf.