I had this recently. i went to the doctors and was advised that had a chest infection, was given some antibiotics and told to increase inhalers useage (both blue and brown)
I then took a week off and now am back running and have a mara on the 16th Oct. I would go to the docs and they would give the best advise, but if your panting and puffing and it dosen't get any better DO NOTrun, there will always be another race and it's better to run it being 100% fit.
I would defo recommend rest, rest and more rest!! If you're still feeling rubbish on Sat then I personally wouldn't run it as you'll feel even worse afterwards, and to be honest there will always be another half to run.
Get plenty of sleep, feed a cold (as my nan says ) and drink lots of water. I try all the drugs going, Beechams, night nurse etc but then I have no paitence for being ill.