As much as I usually hate the term you need...
Physical labour is very different to regular strength training for a few reasons:
- Barbells and dumbells are symetrically weighted
- Barbells and dumbells are static weights
- Barbells and dumbells are designed to be easily grabbed and safely lifted
- Traditional strength training has you doing fixed reps and sets
The bad news is, that when you are moving things around the weights are usually asymetric, they shift as you carry them, are unwieldy and won't move in ways you want them to. Think about a bag of cement versus a barbell, a bag of cement will weigh about 15-25kg, which is about the weight of an unladen barbell, except the bag of cement has no easy hand holds, and will move in your hands, an you'll be moving them all day long.
Rather than join a gym, I'd probably recommend looking into sandbag training. Most plans will be similar to the one just linked, but will be based on high rep kettlebell
like movements. Kettlebells are fantastic for weightlifting endurance, which is why they were so revered by the Russians
By their nature, typical kettlebell exercises build strength and endurance ... in a way that mimics real world activities such as shoveling or farm work. Wikipedia
Essentially you need to fill a duffel bag with some weight, like small bags of sand or lengths of chain and use that as the weight. The idea behing many small weights is so you can load and deload the bag as needed easily and cleanly.
Then you'll be looking at doing full body exercises like the following:
- Goblet Squats (To get a weight into an easy to carry position)
- Cleans (same as above)
- Deadlifts / Straight leg deadlifts (To get a weight onto another platform to make it easier to lift)
- One-arm farmers walks (To get a weight to the other side of the yard)
- One-arm snatch (To get a weight onto your shoulder)
- Shoulder press / push press (To get a weight well above shoulder height)
If you already have some strength training under you (weight) belt, dial the weight back when using a sandbag for two reasons:
- The weight will be far less stable - meaning your small stabilisers will be much more important
- You will want to be doing lots of reps, and I mean lots. When you are in the field, it doesn't matter if you can lift a really heavy sandbag once, you need to have the endurance to do it all day long. So train for endurance over raw power and strength.
Feel free to insert the above along side a traditional strength training program, but what will be important when you are down there is the ability to lift weights for a long time without rest.