Hey RMR, you say you run a 5k on Sundays, why are you doing C25K if you can already do 5k? I might be reading it wrong but I may as well ask
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I'll go ahead and reply as if that little bit wasn't there anyway, my advice is if you are struggling with a particular week then slow it down a bit, my first 3 weeks I was struggling a lot and was told by a running mate that it was good to struggle.
I then told him I was REALLY struggling and he just told me to slow down! You may not realise it, but you are probably going a lot faster or at a much higher pace than you think you are, plod along on your next session, and try to go at a comfortable pace, so that you aren't struggling too much, but still going hard.
There is absolutely no harm in repeating a session or week though mate!, Especially if you are feeling a little ill or weak, it might even be wise to have a couple of days off of runnning, as a beginner it might really help. You will feel healthier and your running should feel invigotated after your extended rest.
I am sure you will be just fine!, good luck