The tunnels are alright i’ve found - Takes a sec for your eyes to adjust though. I had DRL running on the back of my bike, so that kept me visable in the tunnels.
Like others have said, just keep eating and drinking as much as you can, and make use of the water fountains in the villages you pass through. I ran out of water 5k from the top of the Madeline and it was about 38c at the time. I stupidly forgot to fill up my bottles at the bottom of the climb!
Coming down the Galibier and then down the Lautaret on the way to Alp D’huez was by far the best descent i’ve ever done. It’s like being in tron when you’re flying through the tunnels with the lights flashing past!