How funny, I just got back from a weekend conference at Disney followed by a week at Captiva (the island right next to Sanibel). I agree with the drive times posted. Don't be fooled by the mileage, it gets really, really slow close to Sanibel. I'd give it an hour to get from the closest airport (Ft Myers).
The place we stayed on Captiva is about 5 miles from the end of Sanibel, but it would take us 45 minutes to get from our house to the Causeway. I loved the week we spent there, but I'd definitely plan for a low key week because you may get sick of the drive!
If you want recommendations on activities or food or anything, let me know!