I think staying flexible is high on the Black Widow's fitness regimen and to do that I figure she would incorporate various stretching exercises including yoga and Pilates but also ballet and gymnastics.
I just started doing basic or beginners yoga and I find it to be very relaxing since it's not going too fast or I'm doing stretches that are too advanced. It's going to take more than just beginners yoga to get anywhere near Black Widow's range of flexibility but I have noticed that mine has improved.
Challenge for today: To find a fitness inspiration. It could anyone who you admire be it your best friend or Superman as long as they are an inspiration to becoming a more healthier you.