If that shit was easy, everyone would be doing it. Running, for me, is the most challenging cardio exercise both physically and mentally. How do I keep motivated? Like anything in life, sometimes you have to dig deep to find that morsel of inspiration. Many times I'll be running along feeling like I can't go on and then start a dialogue in my mind something like this---why am I doing this?, can't stop now, damn can't breathe, concentrate on breathing, maybe I'll stop up there this time, stop now? no freakin way, what games are on tonight?, am I getting fat?, need to weigh in, ate all those chips yesterday, mmm chips are good, must eat less chips, remember that chick from 12th grade bet she is married to some fat shmuck now, must keep running, I paid too much for these shoes to be idle, why am I doing this?, never take the easy way out, blah blah blah etc.
Running outside is so much easier, but not always possible. At least you have some scenery for distraction. When running at the gym, I can find other distractions including the tv.
Right now I am nursing a mild case of shin splints, and I don't even run much. Like you, I run 2-3 times a week for 3 miles. BTW, it also takes me 30 minutes.