Doesn't sound unreasonable. There is much, much more heat produced when you're travelling at that sort of speed. With the age of your car (you don't say what year/model but it's going to be at least 12 years old...), it's quite likely that the radiator isn't doing as good a job as it might be, probably as a result of mild furring inside and bent fins from stone impacts over the years. Plus your engine may have lost some of it's efficiency. Put all this together and it could be struggling to dissipated the extra heat.