3. Rest, dummy.

I haven’t made this mistake because I’m all about the rest days, but it’s a common one: You do too much, too soon. You run, like, five days in a row and either hurt yourself or burn out in the process. The general rule of thumb is to rest one or two days in between runs (at least for new runners). After a long run or between moderate runs, you can do a “recovery” run (two-three miles), but it should be at an easy, leisurely pace. Without rest days—not to mention cross-training days where you do different activities like yoga or swimming—your body can’t recover enough to give you the proper “juice” for your next run. I’ve also read that you shouldn’t increase your mileage each week by more than 10 percent.