dvl dog,
Stick with your principle that easy days should be easy, that's what will keep you from getting injured. For you, it sounds like you could go a little faster on your training runs (maybe 10 to 15 sec/mile), just to make you a little more uncomfortable, although don't do it on your long day. Remember this must all feel natural, never try busting a nut on training runs, unless you're running with teammates or friends (my team tends to get out of control every once in a while). I highly recommend substituing one of your training days with a speed workout - maybe do speed workouts tue/fri instead of just thursday and then keep the rest of your training days the same. Start gradually increasing your mileage as well. When you tell me you do 3-4 on tuesday, do 4, same goes for the rest of the week. Increase your mileage anywhere from 3-5 per week, whatever feels comfortable to you. I strongly believe that many people have the misconception that running is supposed to be extremely hard and waste your body. So wrong. If you go too hard consistently your body will breakdown, ph levels get messed up, it takes a toll physiologically. The only days where you should be really tired after a run are your speed workout days, and maybe your long day. It sounds like you're doing good track workouts, although what kind of splits are you hitting? It might even be better to do a little less at a faster pace. Any other questions let me, I think you're on track for a good time as well, just need to pick up the mileage a bit and get a lil more speed in there. Best of luck.
Pat