whoohoo! i ran 20 miles for my long run this weekend in preparation for the chicago marathon which is, unbelievably, only 4 weeks away! i was “only” going to run 18 miles for my LSD ( long slow distance ) run this week, but things were going so well i decided i had enough gas in the tank for 2 more miles. i think i’ll probably run another 20 next week, maybe a bit more if i’m feeling good and then it’s time to taper down for the marathon.

since it’s getting close to race time, people are asking more often what time i think i’ll finish the marathon in. the answer to that is pretty straight forward and gives a good opportunity to give a refresher in sports nutrition and glucose metabolism.

despite running for a year and losing what i hope will be around 70 pounds on race day, thanks to my 6’4″ frame and the genetics card i was played, i will still be, er, solidly in what is known in the running world as the clydesdale class which is an endearing term for runners who weigh over 200 pounds.

as a card carrying clydesdale, i burn a lot of calories when i’m running. how many? if i run 10 minute miles ( 6 miles per hour ), i burn around 1100 calories per hour. the problem is that even if i carbo load before a long run, i’m only going to have about 2,000 calories in stored glycogen in the “gas tank”. once that gas in the tank is used up, you hit the dreaded “wall”. you’re done. kaput. obviously, if that was all i had, then i could only run for 2 hours or about 12 miles. i can and will take in as many calories as i can while running, but an endurance runner can only take in and absorb about 275 calories per hour. try to take in any more than that and you’re going to run into, uh, gastrointestinal problems and bring on a case of the runner’s trots ( or worse ).

so, i’m burning 1,100 calories an hour at 6 miles per hour, which is a 4:20 marathon race time, and i can take in about 275 calories an hour which nets about an 825 calorie burn rate. good deal! i can burn some fat! surely i have enough fat to burn! ah, but at moderate levels of physical activity you can only get about 50% of your calories from fat ( and you still need glucose around to burn that fat, once the glucose is gone you lose the ability to efficiently utilize either source of fuel ). so, i need about 800 calories per hour and i can get 400 of those from fat which means i’m going to drain the glycogen tank at the rate of 400 calories per hour.

if i have 2,000 calories in the glycogen tank that means i can run for 5 hours. awesome! if run at 6 miles per hour or a 4:20 marathon race time, all the math works out in my favor!

but what if i want to push it and run an unadjusted paul ryan race time of 4 hours? well, then i burn a little more calories, or about 1,200 per hour. after i take in 275 an hour in “goo” and sports drinks and whatnot i’m netting 925 calories per hour. here’s the kicker, the additional exertion decreases the efficiency of fat burning and i can then only get about 40% of my calories from fat which means i’m burning 555 calories an hour from the 2,000 calorie glycogen tank which means i run out of gas in about 3.6 hours. or about 30 minutes before i theoretically finish in 4 hours.

which is a long way of saying, if everything goes well, i can probably run a 4:15-4:30 marathon without risking running out of gas and hobbling towards the finish line.

by my calculations, i could theoretically run a sub 4 hour marathon if i weighed about what i weighted as a sophomore in high school. that might happen someday, but not this time around 🙂