The 800 calories could have been eliminated by not eating them but I think that's where the diet only approach runs into difficulty. I had already moderated my diet, reducing by a further 800 calories I would effectively be starving myself. If I'd felt hungry all the time I would have been setting myself up to fail. Also, by doing the exercise I got quicker results, a better physique and I felt fitter and stronger which all helped to boost my motivation.
In years gone by people used to have much more active lifestyles, manual work, no labour saving devices and fewer cars etc. Evolution hasn't caught up and as human beings we're programmed to consume 2000 - 2500 calories per day. Anything much below that and you're going to start feeling deprived and hungry which is why diets often fail. That's why I think exercise is key, especially for someone like me who likes their food, works in an office and like most people spends a fair amount of free time slouching in front of the TV on the sofa.
Ultimately I think it's a simple equation of calorie intake vs calorie expenditure. For example, when I had reached my target weight I was able to reduce my gym/running from 4-5 times a week to 2-3 times a week and ate pretty much what I wanted. I do think it's possible to lose weight by adjusting one or the other but in my experience I got faster results that were more sustainable by adjusting both.
There are, of course, people with fast metabolisms who can eat a lot and stay slim but I think they are relatively few. I hazard a guess that if you were to analyse what a most slim people ate you'd find they just eat less and/or expend more energy.
There again, I guess it's different strokes for different folks, whatever works for you personally.
