Come to think of it, since I was living in Hawaii and then Guam at the time, it may have been 3+ years before I ever got onto something you’d consider a freeway (onramps, guard rails and overpasses). There just weren’t any!
But I did ride on the main highway (speeds of 65 plus) on my first road ride. I pretty much had to as it was the route from the dealership to my home.
As others have said, watch your following distances as speed can feel very comfortable and it’s easy to creep up on the car in front. Also watch those transition zones where freeways merge and split, as that’s where you’re likely to encounter drivers changing multiple lanes at the last minute and not watching for a bike.
Use your lane position to increase both your ability to see far ahead and to be very obvious to motorists around you. When starting out, be sure to avoid settling in behind cars that tailgate, as they are more likely to make erratic changes and are often pushing their own limits.
But on balance, I find freeway riding to be less taxing per mile than city riding. People are coming at you from fewer directions and it’s a bit easier to keep tabs on the surroundings.