I've been on two very short cruises on the Star Virgo and a couple of years ago, we were lucky enough to travel on the QM II, that was some experience. We travelled for 31 days from Perth, Australia to the UK, travelling around Africa. It was one of the most romantic things we had done.
We made the booking about 4 months before the date of departure as we found a really cheap offer.
We both felt that a two week cruise was probably long enough. An inside cabin is okay but on a longer cruise you're not getting as much fresh air as you would with a balcony cabin.
We got plenty of excercise by taking the stairs and taking walks around the decks. We ate much more than usual without putting on weight but hardly touched anything made with flour and no rice or potatoes. There's too many delicious and healthy choices without filling up on those carbohydrates.
There was a nasty flu going around and just about everyone on the boat caught it at one stage, we had brought some antibiotics with us and a few other medicines, including sea sickness tablets.
I would not recommend it for people who are prone to motion sickness.