we are as yet far from "peak land" use as far as agriculture is concerned

In the EU 27, I don't think you've made a case for this. We don't have that much spare land.

20-25 million hectares (DoDo) of Russian land of unspecified quality  may provide some biofuels. But you can't just count it all in immediately as half the EU's 10% needs -- as I said above, for biofuel production it needs to be prime arable with the right climate (plus irrigation for maize), to be easily mechanisable (ie flat) and with easy transport infrastructure, and land possessing these qualities needs to be grouped together in the same region to justify industrial investment and to facilitate export of the finished product. There are constraints here that mean it will provide far from 5% of the EU's current consumption, without adding Russia's own consumption in there (since we're envisaging an end-game scenario with little or no liquid fossile fuels).