Strickland is quite, old, right?
He's 73. If he were elected to the Senate in 2016, I think he'd be the oldest non-placeholder to enter the Senate...ever. But that doesn't mean he'd be a one-termer; like others have noted, others have served well into their 90s, and Strom Thurmond crossed the 100 barrier. He could stay in office for a while, especially if at 73 he still has the energy to do something new like this.
We might end up seeing more of this in the future.
People are living longer, and we're getting older statewide officeholders than ever before. It's conceivable for those guys to run for Senate ten years after getting elected Governor.