Do you lose running fitness quickly?

99 percent of the time, I’ll run 6 days per week. I don’t miss much. But there are those occasional weeks when I’ll have to miss 2-3 days in a row. It’s not often. Maybe 3-4 times a year. And when it happens, I can almost always do another form of exercise. Stationary bike, swimming, weights, plyometrics, etc. But when I come back, it feels like my running fitness has taken a hit, especially on the first couple of runs. I go by heart rate, I’m noticeably slower. If I go by pace or power, my heart rate is noticeably higher. Then, it tends to return to normal sometime in the next few days.

With weight training, I can miss a week and come back feeling stronger than ever. But with running, it seems to fall off pretty quickly. Is this normal?