I'd suggest its quite doable, particularly given your experiance. Easiest is probably out through Eastern Europe, down through Syria, Jordan, Saudi (probably on a transit visa), ferry to port Sudan then Sudan, Ethiopia, Kenya. Yes conditions would be harder in northern Sudan, northern Ethiopia and Northern Kenya - coincidentally the south of each of those countries has good tarmac. All three will be fine providing you aren't trying to cross in the rainy season. Dependant on choice of vehicle, how much clearance, etc but lots of local vehicles pottering round are 2wd. Harder route could be Tunesia / Libya / Egypt although as far as I'm aware, these would only be hard if you choose to go out in the desert, I think there are tarmac options nearly all the way, but would limit how much you see. Hardest route would probably be through West Africa, mainly because you would have to look at something like CAR to get across to Uganda. Relationships are deterioriating further between Chad and Sudan, so you would be lucky to get across there. Your alternative on that route would be to go all the way down to Namibia then loop back round and up.