i find i get the best results when i switch things up. maybe go for a jog, play some basketball or do some calisthenics/yoga? something that works the heart, lungs and major muscle groups, but still lets you recover.
and that's the other thing. i find it really difficult to improve or even maintain fitness when i'm not recovering properly. eat right, get sleep, and take days off when you can. i mostly bike for transportation so that can make things tricky but on my easy days i stay out of the big ring and allow myself plenty of time to get places.
this is all just one guy's experience, i'm not an expert in any sense of the word.