Chest encumberance is easy to maintain, as anything like (heavy) armor or backpacks should be taken off before a fight with a low amount of zombies.
Otherwise it’s best to wear armor as most mundane zombies will have very low levels of damage, and kiting with a ranged weapon is surprisingly easy.
And gaining XP in the higher levels is rather difficult as you need to fight harder zombies to train. (I can’t be too confident on this answer, as it more speculative than the others.)