Salutations!
There are 3 grammatical errors. The first sentence: have you ever run in a relay race.
When you start a sentence, you need to start it with a capital letter. This sentence is asking a question, so you need to add a question mark.
Have you ever run in a relay race? (Rewritten accurately)
The second sentence: if you imagine a relay race, then you'll understand how the Pony Express worked.
after a full stop or period, you need to always start the sentence with a capital letter.
If you imagine a relay race, then you'll understand how the Pony Express worked. (rewritten accurately)
Hope I helped (:
Have a great day!