Honestly Pack Marching itself is the best, use that as the base and mess with variables such as speed, distance and the weight you carry, so mabye some high intensity interval training combined with distance work and of course learn to carry more than specified.
Obviously dont go all out from the start, the key is progressive overload so when you adapt to a particular intensity up it a bit. A smart aproach will prevent injurie. Also the terrain you will be marching on wont be plesent during selection. So match that in your training as well, so you want uneven ground uphill and downhill etc. You need a good pack, make sure you take the time to make it comfortable. You need good boots to make sure you wear them in.
If you want to cross train at the gym you will be working on strength and endurance best to use compound exercises such as deadlifts, squats etc. Best to speak to the gym trainer bout that though.
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