I don't think your weight is the problem. Let's be reasonable about this. The best advice is to make a serious, good faith effort to get into physical condition. It sounds like you have done this. So what if you don't have the same oxygen function of a Lance Armstrong. So what if you don't do as well as a Rocky Mountain native. You have done your level best to get into shape. Your guide will just have to accommodate a pace which is comfortable and manageable by you.

According to the BMI scale you are just a shade overweight. This is a tough standard. There are a lot more overweight, nay, obsese people struggling around the mountains trying to hunt elk than you are even close to being. Do what you can to prepare. If you can go early to acclimate, great. But just accept that you have limitations and you have done your level best to train.