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Sounds like your going fasted cardio, which has its merits but also drawbacks.
If your doing a morning run commute with little or no simple carbs for breakfast to fuel it, give yourself an extra 10 minutes or so for an easy effort. Your still going to burn roughly the same energy (circa 500cals for 5 miles) but less of it will be from your glycogen stores so you shouldn't feel so wiped out later.
Ran into work this morning. Been tired all day. Was thinking of running home. Am thinking fuck that. (Is only 5 miles).