Yep, basic physical fitness is key. You'll need a sensible plan to prepare for that. It's not just reaching a particular speed or level of stamina; it's also conditioning your body to cope in other ways. For example, though I was very fit it took ages for my feet to harden up to cope with long marches wearing boots and carrying kit. Some people develop shin splints during army training, because their bodies are just not used to the work, but the army have good support and they'll be looking to work with you to help to make the grade.