Set a timer for 15 minutes and write about yesterday. You can be factual, weave a story about an incident or write as a text message or letter to a friend.
When the timer buzzes, stop even if it is mid sentence.
Count how many words you’ve written, multiply by 4 and you have a measure of your writing speed. That gives you a means of thinking how long it might take you to write a blog entry, an article or even a book.