Roger has nailed it again
A few quick thoughts from a 245 lbs guy who just went through all of this relatively recently
(1) With your weight, watch how you are falling into shallow water. It is really easy to twist your ankle. At my age (45) I begin to notice, these things do not want to heal quickly at all and leave behind some nasty residual, arthritis-like pains.
(2) Loosing some wight in any way you can will be really good for you
(3) If you are having the same problem I had at first with wrist and elbow pains, the good news is, they eventually get used to the increased loads. I was initially suffering immensly at night, especially with some minor arthritis problems i have.
(4) The foot and back pains are mainly a function of poor posture and position on the board. These will go away really fast as you progress. real soon, you will be probably quite amazed as how efrotlessly you can do certain things that used to be such a pain.
(5) Limit your wind speed/wave height exposure at first. Strong wind/high wave combinations are not only a spirit killer for a beginner, but will leave you totally exhausted sometimes after just a few attempts to get on the board and uphaul. That too improves greatly with the technique improvements and the sense of balance you gain with time.
(6) If you are not using shoes, try it. I have water shoos with relatively rigid soles and that has helped me with my initial strong foot fatigue. Off course, it may be a required safety item, depending on the bottom in your area.
Windsurfing will teach you humility, as you body will be defeated by the weather time after time in the months and years to come, and you will be tested to your limits. However, that is exactly what makes it such a great sport. If you persevere, you will eventually get a very rewarding sense of accomplishment. And that is true at any skill level...