I think this is a very noble goal and definitely attainable. I am also a third year, and I just began running recently. I have always been a weight-lifter and that has continued throughout med school. I have managed to be in the top 10 in my class, score 240+ on the Step 1, and honor some shelves. Not to brag (this crap is anonymous), but just to show that it's definitely doable. My attending right now in OB is crazy busy and has ran a marathon in every state! He's running another in a couple of weeks, and he is nearly 60! If he can, we can, too! It's all about time management. Who needs TV and Facebook?!?
I usually lift for 1.5-2 hours a day. Most of your weekly runs won't take this long, especially in the beginning.. I actually have more time to study now that I've began lifting less and running more. You can do your long run on a day off first thing in the morning, get it out of the way, then have the rest of the day to study. When you're are busy, you can often still find 45 mins to an hour for some speed work or a tempo run. You'll find ways to fit it in if you want to - if it means getting up at the buttcrack of dawn to squeeze in a run. I'm on OB right now, and I'm still getting in at least 5-6 miles a day (like I said I just started running)!
Not to mention, I think this would be a great thing to add to a residency app to show that you are well-rounded, motivated, hard-worker, driven, good time manager, health conscious, etc. Not to mention, it gives you something to talk about at your interviews. Ex: What was the hardest thing you've ever done? Greatest accomplishment, etc. Completing a marathon while making good grades in med school sounds pretty awesome to me. It's the kind of stuff I would look at if I was a program director. It sets you apart and makes you unique.
OK, I'll stop rambling. That's just my opinion. You can do it!
