Yes, I goofed. However, believing that 11.4 years is what you'll get in practice is also naive,
Not, it's not your basic conversion error that's the problem.
A MTBF of 11.4 years does not mean that a typical array will have a lifetime of 11.4 years. From Wikipedia:
Once the MTBF of a system is known, the probability that any one particular system will be operational at time equal to the MTBF can be calculated. This calculation requires that the system is working within its "useful life period", which is characterized by a relatively constant failure rate (the middle part of the "bathtub curve") when only random failures are occurring.
You are conflating "useful life period" with MTBF. They measure different things.