You're booking a one hour long window where you can use the fastpass at any time during that hour (and they'll often give you a few minutes grace period as well if you're nice), not a specific time. So if you book a fastpass for 9-10am, you can show up anywhere during that hour to use it. That means even if your next fastpass is from 10-11, and you don't get to your first one until almost 10, you'll still have almost an hour to make it to the next one before it expires.
I would strongly encourage you to book them back to back, and as early in the day as possible. if the park opens at 9, then booking them 9-10, 10-11, and 11-12 is fantastic if you can do it for the attractions you want. As soon as you've used that last one you can use the app to book another, and keep booking new ones as soon as you've used the last one.