I don't know what the state of your spine is, but maybe you'll be able to do reverse sit-ups? You basically lie on the ground, then bend your knees and bring them in towards your chest. So instead of potentially straining your neck and back, you'll be focusing more on your abdominals to pull in the weight of your legs towards your chest.

If you don't think you have the time/energy to devote towards a novel at the moment, I think it would be worth exploring short story or novella competitions that lead to publication. I definitely believe you've got what it takes to get published, and even if it's not a novel, it's still a byline. :)

(sorry Tea for taking over your fitweek!)