I had something like flu come on while at work, but it was more headache and back ache. It was several years ago so I forget the details, but it wasnt winter, so I stopped and lay on the grass several times to recover. Instead of a 70 min ride home it took over 2 hrs. I left work early so I wasnt riding with the rush hour crowds. I think you should drink plenty of water during the day as you dont want to be dehydrated.