The workout you get also depends on your type of windsurfing...
Last Saturday, took my big gear out (165lt board, 7.5 cammed sail) for some flatwater fun, was like 'armchair' sailing...barely raised the heart-rate, but I fully enjoyed it!

Planing fairly fast, leaning back on harness & just using arms for steering...100% gybes. The only reason I'd call it a workout was due to the size/weight of the rig.
Yesterday, 20-25kts & gusty on small gear...woke up this morn & felt like I'd been hit by a bus!