The measurements of pressure- and mixed-mode oscillation frequencies in subgiant stars offer a unique opportunity to probe their internal structures ─ from the surface to the deep interior ─ and to precisely determine their global properties. We have conducted a detailed asteroseismic analysis of the benchmark subgiant $\mu$ Herculis using eight seasons of radial velocity observations from the Stellar Observations Network Group ─ Tenerife, and have determined its mass, radius, age, and surface helium abundance to be $1.105_{-0.024}^{+0.058}$ M$_\odot$, $1.709_{-0.015}^{+0.030}$ R$_\odot$, $8.4_{-0.1}^{+0.4}$ Gyr, and $0.242^{+0.006}_{-0.021}$, respectively. We have demonstrated that simultaneously fitting the helium glitch properties, oscillation frequencies, and spectroscopic observables yields a more accurate inference of the surface helium abundance and hence stellar age. A significant discrepancy between the observed extent of the helium ionization zone and that predicted by stellar models is identified and examined, underscoring potential limitations in the current modelling of stellar interiors. Our analysis confirms that the helium glitch originates from the region between the two stages of helium ionization, i.e. from the $\Gamma _1$ peak, rather than from the second helium ionization zone itself. Within the conventional formalism, this implies that the glitch analysis characterizes the region located between the two helium ionization zones.