Your right, it is pretty much a done deal with me. And I do think it is possible...below is a plausible theory explaining why peoples lifespans could be longer in those days. Remember that an intelligent Creater would logically have created people initially genetically perfect.
"Barring accidental death, one-celled organisms are potentially ‘immortal’. A bacterial cell reproduces by dividing into two where there was one, those two then become four, and so on. Why then do multi-celled organisms die? Individual human cells in tissue culture divide some 50 times and then stop -some sort of pre-programmed genetic limit is reached. Human tumour cells, on the other hand, can be propagated indefinitely by division -the DNA mechanism for preprogrammed cessation of division appears to be lacking or damaged in such cancer cells.
In multicellular organisms, once damaged and worn cells can no longer replace themselves, death is only a matter of time as the function of whole organ systems deteriorates. So even without accidents or disease, there is a programmed ‘upper limit’ on our age, which appears to be 120 years or so as previously stated.
I suggest that our ancestors simply possessed genes for greater longevity which caused this ‘genetic limit’ to human ages to be set at a higher level in the past.
Suggestive evidence in support of this is the fact that in some other organisms (for example, fruitflies), it has been shown that changes in average lifespans can be bred into or out of populations. Most of us also know of individual family lines in which many successive generations all seem to live to very ripe old ages, with apparently delayed senescence relative to the norm. Reports of entire populations (for example, the Hunzas) living to 100+ far more frequently than is the case in our society (in spite of indulgence in tobacco and alcohol) has caused many to hunt for their dietary secrets. However, genetics would seem to provide an obvious answer.
If this suggestion has merit as the major (if not the sole) cause of greater pre-Flood ages, then the obvious question is how some of these longevity genes were lost. The human population went through a severe genetic bottleneck at the time of the Flood—only eight individuals. The phenomenon of ‘genetic drift’ is well-known to be able to account for ‘random’ selectively neutral changes in gene frequencies (including the loss or ‘extinction’ of genes from a population) which may be quite rapid. Also, loss of genes is far more likely in a small population."
http://www.answersingenesis.org/tj/v8/i2/lifespans.asp
(and the same other doesnt write every article, there are several, all with credentials)
Again, we can never know for sure, but this seems plausible. It may have been this above or another reason but I beleive that it happened. (btw, there is evidence for a global flood outside of the Bible)