Ada is definitely more complex than C, but not as complex as C++. If you can draw the parallel between .h/.c and .ads/.adb files, you're halfway to using Ada-as-C. If you then learn the basic procedural syntax and just enough about packages to be dangerous, you can use Ada as a moderately safer C.

You have to learn the type system, task and protected types, and a few other things to really get the advantages of Ada, though. While those aren't hard, they aren't "simple" in the same way that C is simple. But you can learn them piecewise, which is good, as it permits incremental learning of the language.