I've always been rather sceptical of that. Something like Denmark and the immediate vicinity of the great rivers maybe, as permanent garrisons in those parts could still have been supplied without truly onerous logistical difficulties by water. But the deep interior, agriculturally unproductive until the widespread adoption of the heavy iron plough over half a millenia later, with lousy communications and full of unruly natives only too familiar with hit-and-run warfare in the trackless forests ?
No. Just no. Same reason why they never went too deep into the Sahara.