Personally I would suggest more than two months outside doing the long ride, at least three or four on your intended race bike. Trainer is great but at some point you need to transfer that power to the road, and this can affect things like HR response. A longer period of time gives you more ability to build the core strength of riding TT position on the roads and get comfortable in that position, which will interact differently on the road than in a fixed position. You'll be a lot more comfortable on race day.