[bennyelv]

That's the point though - if you get your fitness up then 600 kcal/hour should be possible on the bike and it should be sustainable to do day after day at that volume. Mix in the odd day of doing things super hard at 800-900kcal/hour and you're make an enormous difference to your weekly output.

If your goal is weight loss you should probably skip a mars bar every now and then, but I think you're underestimating what's possible after a bit of training and fitness improvement.