My guess is the 40lbs. I'm 57 and 250. My cruising speed on flat terrain is the same as it was 20 years ago when I weighed 50lbs. less. At that point, I quit smoking, gained the 50 lbs that I can't seem to lose (we'll see with this latest effort!) and the two big differences in my riding are that I can ride farther and my climbing is slower. The first due to no cigarettes, the second due to my weight. My average speeds at the end of a flat ride are the same as always, my average speed on a hilly ride is slower.

You also have to take into account the abuse your body has gone through due to the surgeries and cancer. Your disease and subsequent treatment have probably taken an enormous toll on your body. Try not to be so hard on yourself and enjoy what you're able to do.