What doesn't kill you makes you stronger! Push through the pain, it will go away the more you sail, more you train. Definitely warm up before you go out and spend a good 5 mins stretching as soon as you come in while you're still warm. This should deffo help with the acheing the next day. I used to get the foot/ankle pains when first learning, i think it was down to the big floppy dinghy sailing boots i was wearing. The way you strain your feet to grip the deck of the board because of the thick boots. I suppose this is the same as the way gloves can cause wrist tendon pain. I don't think you have anything to worry about. As previous posts say it's probably due to all the uphauling and getting back on the board, especially if its blowing a bit! I learned to love the battered feeling afterwards, you know you're getting fitter and more adapted to windsurfing! Good luck!