You can not really walk from Rongbuk to EBC, now the EBC is only open for mountaineers, common tourists can not enter...
Besides, your driver will firstly drive you from Rongbuk Monastery to a place where there lots of yak-hair tents set up, mainly for lodging and selling souveniors, and this place is where you'd better start walking to "EBC" , it is not a long distance, but very challangable due to the high altitude, it costs like 1 hour just for a one way walk by taking some short-cut near the main road, full of stones but much interesting; and if you do not want to hike, from here you can also buy tickets for a green bus to "EBC", costing RMB 25 per person