If you take the metro, it is less than 30 minutes door-to-door to reach the Tretyakov Gallery. From Komsomolskaya Metro Station (a few steps from the Hilton Moscow Leningradksaya), take the red line and travel south two stops to Chistye Prudy. Change there to the orange line and continue south two stops to Tretyakovskaya, which is just over five minutes' walk from the main Tretyakov exhibitions on Lavrushinsky Pereulok.
