' muricin here , just worked an extra 10 hours today past my usual 40 hours per week despite today being a US holiday . <| sep |> Don ' t forget Steal My Sunshine by Len . God that was a great song . <| sep |> Broadly , listen to your breathing . If you ' re able to talk in complete sentences without having to pause to take breaths , then you ' re running at your steady state . That is to say , you ' re relying mostly on body fat as your source of fuel , and can keep going for a long time . When you eventually tire , it won ' t be because you ' ve run out of body fat . Fats are slow for your body to break down and use as fuel however , so although they ' re functionally unlimited , you can burn them faster than your ability to convert them . This is the pace you do most of your training miles at though you still need speed work as wellAt the other end of the spectrum is when you ' re pushing hard . You can ' t speak , or can only get a couple of words out at this pace . This is your non sustainable pace , when you ' re burning more sugars than normal . Sugars are the nitro of your body . They give you a big kick , and they ' re full of easy to access energy . You only have a limited supply of these though , and can run out in a race , which is why you can ' t sustain fast paces over longer distances . This is the pace you do short to medium distance races at . And somewhere in the middle , roughly when you can speak in sentences , but have to take pauses for breath part way through each sentence , you ' re running at threshold pace . This is uncomfortable , but sustainable . This is the pace you race long distance runs at , and it ' s also an important part of training , though should not be the bulk of your milage .