Havent read the whole thread so do not know if I am repeating what others have said.
I am 45, a keen amateur cyclist rather than anything super special or competitive. Last summer my brother in law and I left LA on 28th July and took 29 days to get to NY, stopping in motels all the way. For those of you in the US (we are British), you may remember it was particularly hot. The first few days took us through Barstow and Baker and the Mojave Desert to Las Vegas and then to St George and across southern Utah.
So I have ridden the desert in late July early August and I would say go for it, but be very prepared. From Barstow, we took the I-15 to Vegas and beyond to Mesquite where we took the back road over Utah moutain. I-15 was one of the most enjoyable bits of the trip. I had capacity for 7 litres of water (3litre camelback, 2x1litre bottles and 4x500ml bottles in my bar bag). Even though I would start the day with all 7, at no point did I come close to running out. We started early (4am out of Barstow, leaving Baker at 10pm and cycling through the night to the Nevada border) and we made sure that when we stopped we fully hydrated as well as topping off the bottles. Cycling at night was essential not just here but in the mid-west as well where the humidity made it even worse. Get good lights. Enough to light up the road in front and not just those designed to ensure cars can see you. I was lucky in not having any major mechanical malfunction. Paul had a puncture five miles short of Baker at about 12noon (I was already there) and there is absolutely no shade except the odd bridge you go under. Indeed it is only when you stop, and lose the breeze, that you realise just how hot it is. Also, plan not just the route but also the elevations. We got into Baker around noon after a lot of downhill from Barstow and feeling relatively fresh despite the heat. It would have been tempting to plough on but my research had told me just how long a gradient it is heading East out of Baker so we were able to hole up and wait until dark.
I would advise choosing routes with sufficient cars on them. If anything goes wrong mechanically, I would rather be on the shoulder of I-15 than some deserted back road.