Hi Kelly,
I'll share my experience with you. I'm 39 and 5'5". When I was pretty sure I was going to transition, I gave myself a series of little "tests" to really see how "serious" I was about transition. I look back now and I think was I was trying to do was basically say to myself "transition is a long, arduous process" and give myself a few "smaller" tasks to see how I handled it.
In October of 2006 I weighed 186 pounds, the most I ever weighed in my life. A few years of basically eating whatever I wanted (cheeseburgers, lots and lots of candy, 3 vanilla cokes a day, etc., etc.) had packed on the pounds.
I set my goal at getting down to the 140's. I set the strictest diet for myself, one that didn't allow any sugar and was extremely calorie deficient. I am not advising you do the same diet, there are health risks involved in doing that for an extended period of time. I followed this strict diet, taking in less than 1200 calories a day, for 6 months. I noticed within a month that I had lost close to 10 pounds, just off of the lack of sugar. I threw in exercise, 45 min. - an hour a day of cardio only and noticed more weight coming off. Pretty soon I saw 169, 159, and finally in the 5th month 149...the sacred 140's!! Right where I wanted to be!
Right? Well, kind of. I got my body weight down, but something more important had happened in the process. I had achieved something extremely difficult to achieve. The actual losing 40 pounds might not sound to difficult but focusing on a diet devoid of the types of foods and especially quantities you are using to eating is torturous at times.
It boils down to this, I think anyone who has lost significant amount of weight and kept it off will tell you the same thing...IT ISN'T DIETING THAT KEEPS WEIGHT OFF, IT IS LIFESTYLE CHANGES. I am a different person now. Sure, I could go back to eating how I did, I could watch my weight balloon out of proportion, etc. but I would be miserable.
I'm on HRT for ten months now, I lost the weight I needed to before I started HRT so it was just one thing less to worry about. I know many people who started HRT AND were able to lose weight afterward. I think it just depends on how your body responds to the HRT and how diligently you apply a lifestyle change.
I'm 135 today, but I just got out of the hospital so I'm expecting that to go up a little. I'll try to stay just below 140 in the long run. Good luck to you Kelly, you can do it, just stay determined and know how important this is to your overall life plan

Meghan