Great first post, arbboy and best of luck through the process, have a feeling you won't need it, as i think you have your mind made up on it, like i did with my first post on here.
With your history of being an athlete and still to this day keeping up with a lot of swimming, if you change your diet the weight will drip off of you.
Sounds like your being tempted by going vegan initially to give you a kick start and that is great, as if done properly, you will lose a lot of weight from just the diet alone without the exercise which always helps! And you won't feel a lack of energy, or would need to calorie count that much really, you can just listen to your body when it is full.
What I am following is what is recommended by Dr.Esselstyn in his book here:
http://www.amazon.co.uk/Prevent-Reverse-Heart-Disease-Nutrition-Based/dp/1583333002He basically calls his diet the one needed for optimal health and guarantees that if you stuck to it for a lifetime you can basically never have a heart attack and a host of other chronic illnesses, and says it is the best diet for cardio health available. Which is why I am trying to stick to it, as I need all the help i can get to get out of the rut. It is backed up by pretty convincing scientific evidence too. The book is only a 100 pages long, and the rest is made up of recipes to try, so it doesn't take that long to get through if you fancy a read.
I have been pretty ill disciplined despite my good results and know if i stuck to the advice given in that book from the start 100% i would have lost a lot more weight and felt even better. I am now not messing around and touching anything I shouldn't until the end of the year, as I am really fascinated by just how good I could feel on this diet.
The diet is effectively vegan, with no oils, nuts and caffeine. It is reliant on wholemeal bread and pasta rather than the white versions, potatoes, brown rice, lentils, beans, vegetables and fruits. You can eat as much as you want of all of those really, pegging back only on the fruit, i think only 5 pieces a day are recommended by him. You will feel full on it and have a lot more energy, probably, going into your swimming. Something to look into if you fancy a decent read.
Also Dr.Ornish with his 'spectrum' too is good here:
http://www.ornishspectrum.com/proven-program/nutrition/He gives you much more scope to eat more poorly than Esselstyn does, but if you want the quickest results I would go with Esselstyn.
Whatever you settle on best of luck, and here to support you along the way.