You should not commence training again until you are fully recovered. Taking an extra day off after you feel well again, whilst frustrating at the time, will help you get back to full fitness quicker. Don't try to cram in the missed training. Just ease back to where you were, with a few shorter runs, and continue from there. You can not batter yourself to fitness, it is about gradual adaption so that you continue to improve from where you are at that moment.