Man Plus

From back cover Orb Trade paperback May 2011:

Earth is in crisis. There simply isn't enough water and arable land to support the burgeoning population. As the threat of nuclear war looms, the United States looks to Mars for the precious resources that could save humanity. But somebody must actually journey to the Red Planet to effect a solution, and it would be far too difficult and costly to change Mars to suit man; therefore man must be changed to suit Mars.

Roger Torraway didn't plan to be Man Plus, the man who must undergo the surgeries and shaping necessary to enable someone to survive the hostile Martian environment. When he volunteered to serve as an understudy to astronaut Willy Hartnett, Roger never thought he'd actually be called upon. But when Willy dies in a freak accident, Roger becomes humanity's last hope.

With every new step toward becoming Man Plus, Roger becomes less and less the man he was to those around him, and more and more something new, something strange and alien. Nothing about him is spared, not even his mind, which must adapt to being... different... if he is to survive.

Synopsis from Open Library.