I Think I Can Do This!

So, I have made a commitment to doing a big part of my commuting to school this Summer by bike. I've ridden a couple of different routes and it is well within my reach and ability, however, the logistics of this is causing me to loose sleep. I need to transport me, my laptop, books, change of clothes/ shoes, and a lunch and keep these things dry. The ride is mostly on neighborhood and wide city roads and is 10.4 miles each way, the last time I did it I rode down and back consecutively with minimal walking on some killer (for me) hills. I don't mind getting wet, but my gear has to stay dry. I was thinking about something like the Otivia Cargo Cache to also store my helmet, gloves and shoes in while I'm in class since it locks and is supposed to be waterproof. I haven't made any purchases yet, but need to get moving on this quickly. If anyone has any alternative suggestions, please let me know. I'm still a big boy, and since increasing my riding (not eating more), I've gained a few pounds, but I'm sure that will change the more I ride.