The potential energy landscape of lithium borate glass of composition Li3B7O12 has been investigated by the charge attachment induced transport (CAIT) technique. Here, native lithium ions have been replaced by foreign alkali ions, M+ = K+, Rb+, Cs+. All experiments exhibit a pronounced decrease of native ion diffusion coefficients over more than 4 orders of magnitude with decreasing local population of Li+. The energy landscape is modelled by a site energy distribution (SED) with a concentration dependent Fermi energy of the native Li+ ions. The width of the populated part of the SED is found to be 250 meV (FWHM). The conclusion is made possible by a combination of a macroscopic ion replacement experiment with a Nernst-Planck-Poisson modelling of concentration depth profiles measured by secondary ion mass spectrometry (SIMS). Possible generalizations of macroscopic transport theory to match an Onsager ansatz are discussed.