You seem very organised and motivated to do well, so you should do fine.
I find subjects like biology can suddenly become very difficult to revise so I normally switch to a completely different, less 'wordy' subject such as physics/maths for a while and then switch back. You could always do more of something else then catch up with biology afterwards? If not, just learn the basics of bio and fill on details closer to your exams.
As far as destressing goes, it does of course vary person to person. I find doing something that completely takes my mind off work (such as video games/playing guitar/reading) very effective, as when doing things like walking/breathing your mind is probably still stressing out as you're thinking about everything! I just do this, for anything from 5mins to an hour, both to destress and as a reward.
Another thing that could possibly impact revision - are you in a good sleeping pattern? I find going to bed and getting up reasonably early, well rested, makes revision so much easier and more effective.
Good luck!