The fittest I have ever been was when I was in-between my former job and starting my master's (a few years ago) and I was dancing 5-7 nights a week -- first in NYC for 6 months and then in Cuba for 6 weeks.
But I doubt that is what ET has been doing for the last few decades, probably his only dancing were his classes, which he did not do every day, and which are definitely less intense than social dancing.
This article says that at one point he was depressed and gained a lot of weight and was teaching his classes from a chair, but then came back and lost weight and started teaching properly again.
Now that social dancing is definitely gone for who knows how long, I am trying to implement a 1-hour a day intense dancing routine through online dance workout videos and the "Just dance now" game. The latter is much more motivating than just following dance videos as you're in a simulated competition with 3 other people, and they have many song options (if you pay $25/year for full access) so I have found 20-25 songs I really like and whose choreos I want to master (mostly Latin and hip-hop).