I'd also suggest practicing nadi shodhana pranayama with breath retention. There is some evidence that holding your breath in a controlled way exercises your spleen, like endurance divers train for, which injects oxygenated red blood cells into circulation. [1][2] It obviously has an effect on respiratory capacity as well. [3]
There are plenty of resources on Nadi Shodhana -- I'd suggest Iyengar's "Light on Pranayama" -- but I'd recommend starting with a 1:1:1 ratio of inhale:hold:exhale counts through alternate nostrils, then moving on to 1:2:2 and then 1:4:2.