stiff knees

I've been pushing up my run mileage as of late as I'm getting closer to the first duathlon of the year, and last night I pushed pretty hard on a 5k. I felt fine last night as I went to bed, but this morning I woke up and my left knee felt very stiff...almost like I just needed to pop something and it would free up. It's not painful really, but it is noticeable when I walk around. One of my coworkers(who runs marathons) says this is normal for him when he's getting his mileage up during marathon season, so I'm not particularly concerned about it. But I'm not sure what the right approach is to dealing with the stiffness in the midst of tri/du season. Should I back off on the amount of exercise I'm doing, or will ibuprofen suffice? What are the best methods for dealing with this issue?