i’m by far no expert but to me it sounds like your just have ADS. your slowing down b/c your aerobic capacity isn’t there to keep your legs going as fast, as long as your legs aren’t getting tired then it could be some muscle endurance as well.
as for training it depends on what your ultimate goal is, if it’s just get in shape/improve fitness just keep at it and it’ll come with time. one thing i’ve learned is the more training with longer sessions the faster the gains, but you have to keep track of your body. the key is can you do a work out and recover by the next day to repeat it? if you can your not over doing it.