I think 100 pounds in 6 months is very optimistic. But I think 100 pounds in 10 months is perfectly doable. I won't say if it isn't healthy because I don't know enough about what you're eating and how active you are. If you shoot for 1% of you body weight per week lost, you will be at your goal in about 35 weeks. I think this is an attainable goal. It worked for me. To see it laid out was very encouraging - I made a spread sheet with my actual weight at the top of the column and then put the Wednesday dates down a column. I put in a formula that made each week's goal 99% of the week before. It was good to see if I was keeping pace with what I wanted to do. Some weeks were better than others - for no apparent reason - but overall, I was able to maintain that rate of loss. Some people go for small changes, other jump in with both feet right off the bat. You've set a great goal - keep us posted as to how you're doing.
Oh, yeah - welcome to the forum and good luck with this.
Life style change started on Jan 13, 2011. I was going to lose 100 pounds by Christmas.
I lost 93 pounds by Oct 1, 2011 and am holding there for now. We'll see what happens.
New goal: To maintain at about 160
Final Goal: To decide if I need to lose more
Just Keep On Keepin' On