I typically have a weekly workout that resembles this:
3-4 times a week: 35-40 minutes of running/jogging
3 times a week: 40-45 minutes graduated lifting on major muscle groups, on non-running days - quads, calves, glutes, ; running days - arms, shoulders, abs, obliques.
However.
After I run, I tend to be so pumped up that I want to immediately go lift. Conversely, when I lift on my off-running days I feel like I don’t get as much out of it. I’m lifting for muscle definition and overall strength, so I usually do a set of 15-20 medium weight and then 10 of my higher weight ability.
So I find myself wandering down to the weight room more often than not after I run, regardless of whether I have worked those muscle groups the previous day. I know it’s not good to do back to back, but if I don’t have any soreness from the previous day, it’s hard to tell myself not to, since I enjoy lifting more than cardio.
I have been monitoring my protein intake carefully so I can ensure my muscles are able to recover. But how much damage am I doing, and what (if anything) should I be doing differently?
Recovery is not just about nutrition (though that’s important). You have to rest! Recovery time can vary, but it’s vital for everyone. If you’re lifting enough to do any good, then you’re lifting enough to require recovery time. That’s the great thing about lifting: you’re getting the benefit from the rest after the actual lifting. Lifting without sufficient recovery time defeats the purpose and risks injury. Don’t wait till you tear something.
I never hit the same muscle group two days in a row. In my younger days, every other day would be enough recovery. I’m 48 now, and that doesn’t do the trick anymore. At least two days between. I think it’s an individual thing to some extent, but general rule of thumb, skip a day on the same muscle group, at least. You work out enough that your body must tell you. I know when I work out without sufficient recovery–I know exactly how that feels. If you’re not feeling any soreness from hitting the same muscle group two days in a row, perhaps you’re not hitting them hard enough. Work out the regimen that benefits you most effectively, and ignore the rush from working out just for the sake of it when that falls outside your regimen. (And your regimen should mix up the way you hit the same muscle group.)
Also, try mixing in really long runs. After a ten-miler, I don’t feel like lifting anything. 
Yeah, I have noticed generalized fatigue, but it’s mostly when I don’t rest between runs. I will try and change up my lifting but be sure to rest between, thanks for the advice.
Overtraining is a real condition with significant symptoms. What you are doing is not overtraining, what you are doing is skimping on rest and proper recovery.
Stratocaster is right. I just want to clear up that there is a difference between “overtraining” and “not resting enough”.