Taking off a week from running

A week off is great occasionally. Or maybe a reduction by half in volume or intensity or both. I say don't "cross train" but stay very active. ex. hiking, climbing, biking, team sports, yoga, martial arts. Don't worry about trying to replace running with specific amounts of cross training.

Doing nothing, even for a week, is pretty unhealthy in more ways than running paces can show.

Maybe you have low iron?

I'd keep running, but beyond easy. Leave the watch at home, and finish your run when you feel like it. I did this three weeks ago, and ran a couple super easy 30 minute runs, but also a couple 1:15 runs where I just relaxed and went whatever pace fit.

My best advice, don't try to "make it up" the following week by hammering long workouts every other day.