Is vs. Are

Hi!

I've got a quick question regarding the following sentence:

"If you've been playing soccer for a while then you've probably heard that doing strength training on a regular basis, running a lot on the threadmill, and always stretching after a workout is important to perform optimally."

Is it correct to use"is" here or should it be "are important". I'm talking about several things, but it seems more natural to use "is" than "are"...