Timtruro-I live on Longboat Key - about 12 miles long with a good bike lane each side. Good ride is from Bayfront, Sarasota, over Ringling Bridge (the only hill around and only 65ft high! Wide bike lane.), through St Armands Circle and out to Longboat. Total about 32mi.
Legacy Trail from Macintosh Rd parking to Shamrock Park on the Venetian Waterway is nice with a coffee stop in Venice. I normally ride this on Wednesday mornings and if the forecast had been correct, I would have been there now! The ride can be extended to about 42 mi by including Casey Key.

Anna Maria not recommended at this time of year - too many old geezers from up North driving around lost
Also take a look at www.smbc.us website for local rides with the Sarasota-Manatee Bike Club. Send me a PM if you would like some GPX files.