High humidity saps strength and overheats the dogs more quickly. These early wheeled runs are great for building muscle and increasing endurance, however, keep speeds slow. Allowing an over anxious team to race down the trail during the first few runs of the year is an invitation to injury. Selecting a good training trail can go a long way to preventing injury during dirt training. Forest trails with soft footing are wonderful when available.
Dirt roads are also good but hard pack can cause injury to joints.
Even worse, if the blacktop is in direct sunlight, the surface can be hot enough to burn pads. Common sense and patience go a long way toward making these early runs injury free. By the time the first snow flies the dogs should have a pretty good base of training miles. Or at the very least the mass confusion, characteristic of early runs, should be behind you. The dogs and you should have a routine down.