I seem to build up saliva while running and start to have trouble breathing?

Question

i've been running 20 years, lungs, hips, knees, okay run about 20 miles a week, stride has shortened, speed slower as to be expected. the thing that stops me is that i start to fill my lungs with fluid, i start to expectorate, but once it starts it doesn't stop and eventually i have to stop running, why does this happen and what is the solution ?