I agree with MV. There are always a fair few old bikes around at TT travelling at quite slow speeds, so don't worry. All I can offer by way of advice is from my own experience both on a bike and in a car. You look in your mirrors one second and there is nothing there and the next second a stream of sportsbikes fly past you and you wonder where they came from. So just be extra vigilant, especially on the unrestricted parts of the course where there could be people going 3 or 4 times as fast as you.