If you have been diagnosed with pneumonia over the hols you cannot possibly be fit enough to go in no matter how much you would like to. I think you should be absolutely honest about the situation but make it clear you are eager to be there once you are recovered. If you go in and cannot do the job that is what they will remember. Some years ago i had pneumonia and was in bed for three weeks and I needed it. You will have medical certificates so the school will know you are genuine. Obviously it is your decision, but do think carefully, and if this falls through there will be other opportunities. Teaching is a job you cannot do effectively unless you are well. If you are not going to be well give them as much notice as possible. Most important, take good care of yourself and please let us know how things turn out.