I'm planning my first ever bike tour for April. So far my plan is to cycle around Salt Spring Island and a few other Gulf Islands for around 7-10 days. As I'm looking to do a more laid back trip, I'll be doing no more than 50km a day. I don't have a touring bike, so I'll be using a Specialized mountain bike. I have a few questions regarding gear:

I have two Ortlieb packs, but I'm thinking of tying my 60 litre backpack to the top of my bike rack for extra storage. Will this work well?

What spare parts should I bring in case of bike issues? My bike uses disk breaks fyi.

Can anyone share some of their first time bike touring experiences? Are there some things you wish you knew before you set out?