I've always said the achilles heel of SUP is paddling upwind. Ya, sure no problem paddling in light wind, but when we're talking 20 kts plus, then it's time for plan B. There are a few out there who insist that stand up paddling means never sitting or kneeling. To me there's a certain point where common sense steps in. A friend once commented while paddling against 30 kts of wind "be a man, stand up." Well, I got to our spot in about 20 min, he took 50 min and was fried.
The place I downwind in Seattle has no legal parking, so you have to paddle upwind 2 miles to get there. Unfortantly the best waves occur at about 27 knots, a southerly, thus for me a haul to get to the start point. I usually follow my kayaking roots and sit down and use a canoe stroke which works well. Another pov is to paddle prone, like a traditional surfer. As easy as it looks, it takes awhile to get in shape for paddling a few miles or in heavy wind or waves, but it is great exercise.
Here's a shot of Chuck Patterson prone paddling in heavy wind at Steamer Lane a day before the Surftech Shootout in 2010.