Mika, you'll be fine. As stated, a 50-50 mixture will keep you squared away. Start the car, waste some gas by letting in run 5 or 10 minutes or so & you'll be good to go.

I have actually popped the freeze plugs out of a Merc. Cyclone in my youth.
Some freakin' cold day in Milw., started it & took off right away. 5 minutes to the freeway. About 10 miles down the road when I notice the blower is pumping out only freezing cold air, the temp. gauge has pegged to the "H" & the car just stops.
Eventually, after being rescued by a kindly woman while I was walking to the nearest exit I had the Merc. towed to nearest Ford dealer. 24 hours later, after the plugs were re-installed, it was back on the road.

If you are overly concerned, stuff a piece of cardboard in front of the radiator. You needn't cover the whole thing, in fact, that could be bad, but you will get more heat out of the blower.