I would imagine your weight loss counselor was envisioning a slow lazy trail ride when he/she advised you that riding is not a viable exercise for your post surgical regime.
I had no particular interest in riding for at least 4 months following surgery. Healing, learning to eat the right foods and portions, fluid intake, dumping syndrome and vertigo were big factors for me. Many times I would simply over do it and have to sit or lie down on the floor, no matter where I was. That was not fun when it was in the middle of the market while shopping with my children. Having experiences such as that on a regular basis did not make me want to climb up on my horse. As the weight came off I really had to develop a sense of my body all over again. My sense of balance was really affected and once I was back in the saddle it took me quite a bit of time to regain my confidence and find my seat. I was very surprised to feel this way and ended up having several lessons with a pro to help me develop my seat back and work on balance exercises. Basically, I felt pretty crappy for the first 6 months post-surgery.
I had no support group here and hope that you will be surrounded by good friends who can help you through and that you have a positive experience. Looking back it was the best thing I ever did in my life and it has been 5 years now.
However, I did end up making my riding an integral part of my exercise regime. As a long distance rider, I will agree with your weight loss counselor that simply riding will not provide you with adequate exercise for a post surgical regime. I can go all day on a LSD and know I am not burning too many calories just enjoying the trail with my horse. So, you if you plan on spending time in the saddle and want to incorporate that into your exercise regime you will need to step in up. This will be great for both you and your horse.
When you feel like you want to ride, start taking small walks with your horse in hand. Extend those walks as you begin to feel better.
First, discuss starting a program such as this with your weight loss counselor or physician before you start. And, most importantly, start slow. Keep it slow and plan to work on just walking for the first 30 days. invest in a pair of riding/trekking shoes and pair of riding tights. I use the Ariat Waterproof models and love the Irideons. For every mile (or 20 minutes depending on how fast your horse walks) you ride your horse and walk alternatively. Start with walking, mount and ride, then walk, etc. If you can't mount without a mounting block at this stage, make sure you can lower your stirrup to the point where you can get on or have a buddy with you for the familiar "butt push up". (Keep in mind that in several months you won't need that push up anymore!).
When you can walk 2 miles and ride 2 miles in an hour you are probably ready to incorporate a bit of slow trotting. Turn your walking time into "power walking" time and slow trot or jog your horse. Post or rise the trot when you are comfortable doing so. Alternate walking and riding and continue to do this until you are ready to extend your mileage and riding time. There will come a time when you will be able to do a nice little jog and lead your horse while it is trotting beside you.
The next step for me was to add some ascent and descent to our work together. I was not able to do the 50/50 at first on the ascent. But, I walked up hill as much as I could to begin with and my cardio kept improving along with my weight loss. I also carried a plastic bag with us and turned our ride into an clean-up effort and collected trash along the way. This required mounting and dismounting which uses extra calories up too and is great for the leg muscles.
I can only comment about my own experience, but once I was half-way through my weight loss (8 months post) I felt like I was ready to really start pushing my riding exercise. Posting the extended trot for a long period of time is really difficult and hard work. After 10 kilometers or more, you will agree. On my exercise scale it rates right up there.
I found that my trail riding and enjoying the outdoors as much as possible really added to my life style program. Not only did I become fit, a better rider but my horse is rock hard and better conditioned than any other horse here at our farm. I have to smile knowing that no one can keep up with us!
I still like to do those LSD rides but I know that if there is no sweat dripping down the back of my neck that I am not burning enough calories to consider it proper exercise. Also, I still remember the day that I realized that I would never have to use a mounting block or get "butt assistance" again.
Go for it and good luck. You'll reach your goals and improve the quality of your life beyond your wildest dreams!
Ride long and prosper.