How much do I want that Snickers?

I was just mulling over one particular 'theoretical' benefit that might be gained by slow long runs. In my current case it is in the early taper stage before a marathon, but it applies to just about any time. One aspect that I thought would be interesting to guesstimate is what might the 'weight-loss' benefit to marathon improvement time be of a single run, since I don't have a 'ball-park' figure in my brain.



For me - at my last fast marathon I was roughly 60kg and ran 3:05 (to a first approximation). Assuming I maintain the same aerobic ability and just got rid of some 'mass' (which for the sake of this calculation we will assume is fat) then one source claims roughly 2.5 mins per kg lost (with all the usual 'ifs and buts'). I am fairly confident that this could apply to me since I still have a good deal of abdominal fat. If you are skinny as a rake - don't read this: it does not apply to you - go look for improvements elsewhere!

Fat has an approximate energy content of 9 Cal/g and for my normally-clad training run (multiple layers and heavy shoes) 1 km uses roughly 70 Cal. So (big 'if' here) one was to fuel all of the run with fat that would equate to ~8g of fat lost per km. Putting that weight loss into the '2.5 mins per kg faster' gives ~1.2s improvement. Or, for a 20km run the 160g of fat that might (immediately or eventually) be used gives 24s improvement in time.....

I know there are many 'ifs and buts', but I would not have 'ball-parked' the theoretical number for the 'potential single-run induced weight-loss related improvement' anywhere near that. If forced to guess I would have said something about ten times lower.....

Of the 'ifs and buts' the critical one is to avoid consuming more calories as a result of the longer run - that would stuff it completely. Very slow running would take the maximum from fat whilst running and preserve more of the carbo stores such that eating normally would top them up again. Going faster would mean achieving only a partial refilling of carbo stores after the run - i.e. one would gradually pull-down the carbo store and be living generally on empty....Either way the fat gets burnt - during the run or generally after the run (except possibly with a weaker, more hungry feeling...).

I like the idea of generating the marathon-time cost of foods. For me a Snickers costs at most 4s (this is an 'at worst'-calculation predicated on the basis that a Snickers contains no useful nutrients and just gets turned into fat).....

For me the cost of a treat is, currently, not measured in £ and p but in seconds! This really is a 'First World' problem....