Biking is very good for endurance. If you mix in flats and climbing you get both a aerobic and anaerobic workout. Throw in some 30 second sprints every half hour to top it all off.

Beware of the bonk. You will have to increase your carb intake so you don't.

You also need to make sure you don't run low on sodium. A sports drink like GatorAid or Cytomax will do the job and control lactic acid.