My aerobic exercise calendar works on a 3 day rotation, with one day off each week.
The rotation is run / elliptical / stationary bike.
Sometimes the elliptical or the stationary bike can get displaced by other activities. For example - if I'm camping out and we are going to hike 10 miles (either on purpose or just through the course of the day) - I consider that enough exercise. It may or may not be the perfect 30-40 minutes of aerobic activity, but it's not like I will have the energy to complete a half hour bike session once I get done.
Today is run day. Run day is my favorite. It's a chance to get outside, enjoy nature, and run. It clears my head, refreshes my soul, and does wonders for my body.
Today also happens to be perfect run day weather. In the midst of almost daily rain this week (and next) we have a day when it will be sunny and 70.
It's run day and it's Friday. It's going to be a magnificent day.