I recommend that for long training rides you carry:
1. what you need to fix a flat (a pump, spare tube, patch kit, tire levers)
2. Tools for simple repair and/or adjustment (chain tool, spoke wrench, allen wrenches - or a multi-tool that contains these things).
3. Plenty to drink and some source of quick energy like a Powerbar (unless you know you will be riding past places to buy this stuff.
4. Money for a phone call or a cell phone.
Most of this will fit in a combination of a seat pack and bike jersey pockets. If you choose to carry water or some other drink in a Camelback type pack, most have pockets for additional storage as well.
I find that when I have all these things, I rarely need them. It is when I don't have them that the need usually arises.
Have fun and enjoy the ride!