You're on the right track - upper body strength and cardio. If you can run, plan on reaching a minimum of 5 Km in 25 minutes.
You might want to add in a little ruck march work - i.e. hiking with a pack. Work your way up in weight and distance.
If you're looking for other general infantry related skills, here's a few that might help if you have the time in advance:
Map and compass land navigation.
Get your driver's license.
Basic first aid training - St. John Ambulance.
Rifle shooting - AR-15 is perfect since our service rifle is the C7.
Basic camping - keeping dry, setting up camp, cooking for yourself, knots and rope use, keeping yourself clean in the woods, all are precursors for field training.
Hunting - especially stalking, couldn't hurt.
Just what I could think of off the top of my head. Best of luck and have fun.