Rath
Today Will Be Different
Honestly, I wouldn't worry about the physical aspect. You could be the most fit person there, but if you don't have the mindset, or the will to succeed you won't make the cut. I can't speak for the UK, but in the US, the military is more interested in people who have that mindset. But I'm sure the Royal Marines is no different. I guarantee that they want people who are determined to succeed.
It's easier to develop a physique, but near impossible to develop a better mindset.