Agreed with the other two, get your pack weight down and the plan DOSS laid out sounds great. Don't be satisfied until your pack weight is under 40 pounds, the closer to 30, the better. BUT, that's definitely not the solution.
I too am an overweight backpacker, always have been. And I totally know what it's like to be lagging behind huffing and puffing. Sometimes I'm in my groove and I slam it and stay ahead, sometimes even far ahead, sometimes I'm way behind. Some of my backpacking partners hike at a more reasonable pace, others are speed demons and I feel like a big slacker. Moral of the story, I know the feeling.
I don't like exercising in gyms and on equipment. Not to say you shouldn't do it, I just know I won't. But you do have to find something to keep those muscles going through the year or it will always be so hard. The problem I've found is that I hike hard Spring through Fall then not much in the Winter. And to make it worse, in the spring and fall I'm usually in the desert without a lot of elevation change, generally easy hiking. Then when I hit the mountains in the summer, the big elevation changes kill me until a month or two in. My goal this year is to make sure my hill climbing legs are ready for the big elevation before I need it. If you can day hike, or climb a local hill on a regular basis. Do it. If not, I feel a stair stepper is better than just walking or hitting a treadmill. Anything that not only works those specific muscles but hones your endurance is key.
A couple other things that help:
Do other things that motivate you to get out more. For me, posting my pictures and stuff on the internet was huge. A few years back, we bought a house and got a puppy and I barely hiked or camped at all for almost two years. Then I snapped and realized I was letting life pass me by. Getting involved online, posting my pics and getting ideas from others seriously drove me to kill it and camp as much as I could. That process is the root of why this website even exists. If I never shared my experiences, I wouldn't be nearly as motivated to keep making more of them. Another little thing I do is keep a private google map of every place I've ever slept outdoors. It's exciting to come home and be able to put a new marker on the map and actually motivates me to go to
new places.
And last, diet is huge. Everyone who is happy with the standard american diet, you can stop reading now. This is highly unconventional dietary advice.
I have big reservations about preaching my diet to anyone, but at the same time, I feel like a dick for not sharing it, especially with someone that I can totally empathize with. I feel strongly that anyone struggling with their weight should consider a paleo/primal diet, or at least something close to it. I started living primal in November of last year and I've already lost almost 50 pounds. And keep in mind I've had a bum ankle for the last 8 weeks and haven't done a thing.

I wouldn't be the least bit surprised if I was down 100 pounds sometime this summer.
I feel better then I've ever felt in my life. I don't eat grain, sugar, sweeteners or other processed, unpronounceable crap. I eat limited dairy, I eat lots of meat, lots of veggies, fruit, nuts and berries. I eat like mankind ate for millions of years before the agricultural and industrial revolutions wreaked havoc on our systems. We hunted and gathered. We didn't manufacture corn dogs. It sounds crazy when you're coming from the standard american diet, but it has been one of the best thing I've ever done. I have more consistent and pure energy than I've ever had and I feel so good. When I started, I thought I would do it strict for 30 days and then go lax and cheat on the weekends. But what I've found is that once you eliminate certain things, you actually feel what they do to you and it's just not worth going back to them. I still have a beer now and then and I've come to like wine quite a bit. And there are some things that I'm not as concerned about cheating a bit with compared to a big no-no like grain. It's complicated. But in my opinion, it's a
solution, not just a band-aid. </soapbox>
One of my favorite sources for Paleo/Primal information is
Mark's Daily Apple.