I don't run outside much. I do run on the treadmill. I notice the first 5 minutes are the WORST. When I power through them and make it to 10 minutes it becomes a gun challenge. Then when I get to the 10 minute mark I think to myself why not go for 15 minutes? And so on and so forth. I usually stop at 15 unless I'm feeling very motivated and go to 30 or an hour.

So maybe timing yourself might be helpful.

But honestly I find power lifting to be an efficient use of my energy. I work out practically every major muscle in half an hour and then I'm exhausted.