I wouldn't till til you're ready to plant.
Don't bother with the Miracle grow soil, just get bags of compost or manure. Spread that on top, then till, mixing it in. After a couple years amending the soil like that, you will not need to add compost every year, and definitely won't need the Fertilizer. It will be way better than the Miracle Grow soil could ever pretend to be.
Also, ALWAYS plant a winter cover. Oats and peas is great. Keeps the weeds down. Mow it short, leave it there, put the compost on top. Make your own compost, better than store bought bags.