Agree with all the above posters. I always used to worry about being "full figured, sturdy, big-boned" etc.
Got over that real fast. No one really cares! Everyone on a cruise is there for fun, meeting fun new people, eating great food, seeing great things.
Some of the ruins have a lot of steps and can be rough on knees (on anyone!), so you might want to avoid them. All water tours are fun and there are people to help you on and off boats.
As my wise grandmother once said, "What makes you think everyone is looking at you all the time?" That put me in my place!
So go - have a ball - don't worry about anyone's opinions except those you care for.