You can improve your ability to burn fat, resulting in improved endurance and more energy at the end of a ride. To increase fat burning - ride long, slow rides without a lot of carbohydrate. For breakfast, eat some meat, eggs, orange juice, no carb loading such as cereal or fruit. Go for a very easy, long ride of about 3 hours - it is the best for fat utilization. Another suggestion is to not eat anything until an hour into to ride.
Crunches are great for the abs and lower back.
For the legs, use the bike. Shift 2 cogs harder to do this. Push and pull through the pedal stroke - think about it as you ride. Repeat this 6 times, once every 5 minutes. This gives you time to recover between repeats.