I was the same! (And still am, honestly. I'm not sure drinking two beers and eating two slices of cheesecake before a run was in the training... But oh well!) Don't psyche yourself out! I was super nervous to start and convinced myself it was too much before I even started - but after the first day I felt amazing! Don't try to push yourself too hard and don't think that running equals full-out sprinting. I think of it as conversational jogging. If I'm running so fast I can't catch a breath to talk, then I slow down to being at a comfortable pace.