It ain't hard...
Bit of self respect, shed loads of realisation on the part of the individual that they are in the Bosses Armed Forces and don't work for Tesco et al... Oh and a wish not to look like a sack of potatoes while wearing the uniform.
Go for a wee jog every day, anything between 2 to 4 miles, if that starts to get boring then use lamp posts to liven it up and do a quick sprint followed by a steady jog between them or you could even snap out a quick 10 press ups, burpees or even the gym talibans fav 'bastardos' every other lamp post on a 3 mile circuit you'll soon notice it! An hour a day is all it takes.
As they say, have a jog or some other phys rather than a pie!
They should make the Armys version of training/testing tri-service where possible. The only exception I can see is on board ship, and thats simply through lack of space.