as other's have said - there's no need to be setting a final target figure as yet. Break it down into smaller chunks. Be that each half stone award, or, as I did - find something to aim for, and being realistic, work out how much i'd have to weigh to be happy for that date.
My first one after joining in December 08 was to get below 20st (which meant having to loose 7st8.5) in time for my brother's wedding in August 09. I was being realistic - i knew starting when i did i was never going to be a healthy weight by then - but i managed my target (in fact i got my 8st award the weigh-in before the wedding). My next was to get to a 10st loss by halloween that year - made that too. The next was to be at 15st by my birthday at the end of March 2010 - i got there mid-March. Never once did i starve myself or anything along the way to get to these targets - i just worked at sticking to plan.
Looking at the big picture early in the journey when you have a lot to loose can be quite depressing - it always seems to be soooo far away and like we will never get there. breaking it down and concentrating on mini-targets along the way makes it all so much more easier to keep focus - and keep moving in the right direction.
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