leelee - Hi there and I'm so happy for you that you are considering surgery for your weight issues. I can't really give any advice with your fitness question except to say that I have lost around 17.5kgs so far in my journey and everything is easier for me. I've never participated in a half marathon but I can only imagine that since you are doing that now, if you lose say 50kgs then it's gotta be easier, than how you are feeling now - think of your joints!
Please don't talk yourself out of this great opportunity, I'm so glad that I have had my surgery, yeah it's not easy and there are some things that come up along the way but overall it's been great.