Hi Jez, sounds like you have been skimping on those cans of HTFU I prescribed!

But seriously, that is a really good progression in 4 months. It is good to hear that you are sailing so well!
Your fitness now will partly depend on how fit you were when you got into windsurfing - were you used to hard exercise? Four months is not that long to really shape up if you are just windsurfing a couple of times per week.
And what else do you do to keep fit between sessions? For example I ride my bike to work - which is about 50 mins each way (with hills) and do yoga, interspersed with some pushups and chinups for the arms.
I agree that stretching is very important - before and after. I agree with the comments about death grip - having done that myself - try to commit more to the harness and grip the boom lightly. I also agree with the sail tuning comments from decrep - not enough downhaul can play havoc with sail handling and make an otherwise good rig feel heavy and make you feel like you were fighting it the whole time.
As a dietitian and vegan I would caution about too much protein though. Most standard aussie diets have way too much protein (a major cause of osteoporosis). I think that you can be very fit and have good stamina through regular training and not need to take supplements or gorge on meat and dairy. I'm no Arnie Swarzenegger, as you know, but have sailed 230km in a day - just through being reasonably fit and having a good diet.
Massage also should help, I can recommend a great masseur who comes to your home for $60/hr (just pm me).
But also, the early learning stage is physically really demanding - you are working really hard, falling in all the time, water starting, every thing is harder than when you get more comfortable and have more finesse.
But the main thing is to keep having fun!