Exercise is good, though! You don't want to miss your exercise targets, whereas writing is... while we love your writing, it's still a luxury rather than a necessity. };)

You could always reboot one of your series, maybe the problem is that you wrote yourself into a cul de sac and trying to figure a way out of it is draining your willpower? It's easier to write when you have an idea what's going to happen, as opposed to staring at your subconscious expectantly... And having your subconscious go "Gimme a few minutes! Almost... Okay, I got nothin'!"