"Truly ridiculous" is anything you can't bike without reaching your personal "I'm gonna die now" limit. For me, I'd set that at about an hour of riding, which would let me acheive the average commute distance on a good day (26km/16mi). But it's a personal choice. Obviously, as the distance increases, the extra time required to bike it instead of driving it increases as well.
I totally understand the stones it takes to join the cars on the road. My route has no bike lanes at all, and is along the biggest, busiest roads in a city of 350,000 people. But I've been riding my bike around town for 15 years now, so I'm pretty inured to the horrific driving that goes on around me. On the other hand, you are correct, I'm not from the US (rather, from that cold neighbour to the North), so at the very least, I don't have to worry about being shot to death when some jackass behind me gets pissed off that I've slowed him down.
If you are going to try this, there are three things I highly recommend:
One - get a rear-view mirror. It's invaluable when you want to change lanes, and for keeping an eye on the cars behind you that might cut you off at that right turn ahead.
Two - practice vehicular cycling, and take the lane (ride in the middle) when it's necessary to do so for your safety. Vehicular cycling means that your actions are predictable to the cars around you.
Three - if you're really having problems, either mount a camera on your helmet, or mount something that looks like a camera on your helmet. I was astounded by how much more room I was given when people thought they were being recorded.
As far as temperatures, I'm one of the lucky few that experiences days as hot as 100F (39C) in the summer, and -30F (-33C) in the winter. I get both extremes! I am fortunate enough to have a shower provided at my workplace, and I recognize that.