What's Tarshish? Tarshish is the place Jonah was running off to when he refused to follow God's call to Niniveh. (Jonah 1:1-3) Here I'm taking it as a symbol of our own escape from God's calling and God's will. Mainly because this has been in my head a lot lately: "I'd rather go to Tarshish!!" No really - I think one can get there really easily. It's easy to say, "Lord, I'll do whatever you ask me to." But then a situation comes that you really did not want to have to deal with, ever. And in my case my reaction was: NO, I don't want this, I'd rather escape and not have anything to do with this.
Saying no is not really the right option. The image with the plow and looking back is from Luke 9:62 where Jesus says, No one who puts a hand to the plow and looks back is fit for service in the kingdom of God. Jonah was on the wrong way when he got on the boat to Tarshish - he ended up never getting there because God sent a storm to stop him. It's better to get off that road to Tarshish - also because God never gives us anything that is too hard for us; he goes with us and equips us!