Originally Posted by Marylander

Elite endurance athletes spend 20% or less of their time on high intensity efforts. The rest is spent at easy paces (which, for them, are likely much faster than regular folks can do even at their top effort level but they're still easy for the elites). Whether or not a full on off day is required is up to you. I never take one but also very rarely get sick so long as I don't push the intensity too much (literally, I've missed 5 days in about 7 years and that was due to one incident with kidney stones and surgery to remove them). If you don't beat yourself up all the time you don't have to have rest days (or regular periods of being sick).