GPP and work capacity are related concepts. The names give away exactly what they do. GPP = general physical preparedness. Work capacity = how much work you can actually do. Raising your fitness level through GPP of course increases your work capacity.
Work capacity itself is broken down into several components. Not just absolute strength , but endurance, speed, power, etc. This is where your goal becomes important. For instance, you may be able to lift 200 lbs. for 5 sets of 5. Work capacity is expressed as 25x200 lbs. or 5000 lbs. On the other hand, your goal may be to lift that weight as many times as possible within 2 minutes. You'll still come up with a number of lbs., but you'll have to rely more on speed and power rather than absolute strength to obtain your figure.
It's easy to guess now that that way to increase work capacity is to focus on the area that's the weak link in the chain. There's lots of ideas. Sled pulling is a great beginning. You can add in mini-workouts once or twice a day 5-6 days a week where you just pick 5-6 bodyweight exercises like, push-ups, squats, burpees, shadowboxing for 2 min. whatever.
Maybe it's the amount of work you need to do within the workout period takes the endurance out of you. You can try interval training. Intense sets followed by short periods of rest. For instance: Front squats 1x6, rest 1 min. SLDL 1x6, rest 1 min. Step-ups 1x6, rest 1 min. Lunges 1x6, rest 2 min. and repeat the circuit for 4 complete intervals Do this at the end of your workout. This will help you accomplish more work in a shorter period of time.
Another way is plyometrics. The idea here is similar to the other methods. You're stimulating the CNS with different techniques that will enable you to transfer this to weight training. A good article about this is
Plyometric Ability - React Like A Cat And Explode Like Lightning.
One thing I've used is Sheiko's Preperatory Program. The idea is to improve your conditioning through high volume to be able to handle a period of heavier training. Here's the link:
THE PROVISIONAL MONTHLY PLAN - 1. Pardon the poor translation but something like this will def,. get you ready to sling more weight that you have previously.