The associations and causes to fatigue in general comprise endless lists. Hard to say what caused your symptoms if you believe yourself to be in excellent health with the best possible stamina. The simplest explanation would be that you were simply overexerting yourself thinking that you should EASILY be able to complete the run as you ve always used to do...but the truth is that if it s been awhile since you ve done anything quite this strenuous, that at your young age of 70 (and I truly believe that is still on the younger side of the mountain given the fact that human beings can probably live out to about 150 at this point with technology and medical procedures that we have) it still takes much more effort on a more regular and consistent basis to maintain the same level of conditioning and stamina as it did when you were 65. I believe that is the most likely scenario. However, at your age you must concede that heart disease and the risk of a heart attack, especially when performing heavy physical exertion are at elevated risk. Therefore, I believe it would be prudent and even a good idea to get checked out by your doctor before doing this sort of activity again. Make it clear to your doctor that you wish to continue and even become more active if possible so he/she just doesn t come back with something like, Oh, well at your age you should just be taking it easy..... I don t believe that s any type of solution. What you re looking for is truly a RULE OUT of active cardiac or cardiovascular disease. If you re clean then, go for the gold man. A 67 year old man just completed over 6700 consecutive days of running at least 4 miles daily without missing a single day to reach a total mileage of 24,091.15 miles which is the circumference of the Earth. This was his stated goal over 20 years ago when he began running. A senior version of Forest Gump! Live long and prosper with cross country skiing....I think you re exemplary to anybody who knows you! Lemme know what happens.