Hi Rachel, I'll tell you my story about Glandular fever so you can see how serious it can be.
I've been running and competing in triathlons for quite a few years now, last year I did an Ironman and various other races and was fitter than i'd ever been. During the winter i was feeling a bit more run down than normal and training felt like it was a struggle, after a few colds I got a really bad fever over xmas and I haven't recovered since.
After a few trips to the doctor I found out that I have Glandular fever, my doctor told me it can last more than a year! I've gone from being able to run marathons and extreme triathlons to not even being able to run round my garden without making myself feeling ill.
You really need to rest and recover properly because its a horrible illness, not only does it make you feel run down it can make you feel really miserable and depressed. Glandular fever has also been linked to more serious conditions like chronic fatigue syndrome (ME).
If you google the illness you'll find plenty of information to guide you, if i were in your position i'd wait to you are feeling 100% and then start training really gently, a few weeks off might save you months or even years in the longrun.