If that's all it got up to, you're probably fine. If the ambient temperature was already cold and/or windy and the engine hadn't been driven long enough to get the oil hot, cooling off that quickly isn't abnormal. One of the benefits of an engine-driven fan is that when the belt lets go, the fan freewheeling in the breeze coming through the radiator will still circulate some coolant through the engine.

Any idea what took the belt out? Was it just old, or did it fail catastrophically? Recheck your tension in a couple days and make sure you aren't too loose or too tight.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)