Your question should be:
Could you please correct the following sentence?
and the sentence(s) should be:
Today I went running. It has been (or It's been) such a long time since I last went running.
And a tip?
Try inventing a couple more sentences following the same format, and repeating them to yourself.
Today I *****. It' been such long time since I last ********.
It'll help fix the construction in your mind.