Lead Butt (legs sinking)

Much as I'd like it to be "Buns of Steel," it's looking like legs and butt of lead in the pool. They're definitely sinking. Background: I'm returning to the pool after something on the order of 15 years (I've only completed 2 workouts in this, my first week back). I swam competitively in high school, and a year of Masters in college. I was reasonably fast in high school but since I went to Kenyon College, I was never going to be fast enough to make the workout squad, much less the competing team, so it was Masters or nothing. I was pretty much fat-free when I was swimming competitively, but now, while still not fat, I've gotten a little more well-marbled and have a little fat around the middle and upper body while my legs remain quite lean. I was a freestyle and fly sprinter, and I'd love to get my 50 free back under :30 and my 100 back under 1:00. Right now, I'd just love to be able to do a 50 or 100 fly without stopping. I've always had a fairly weak kick. I've stayed pretty strong with rowing and martial arts, so my wind and endurance haven't been too much of an issue. I never had trouble with my legs sinking in the past, but now it seems to be an issue. Here's the question: Should I be worrying about my sinking leg problems at this early stage? I'm really just getting my stroke back--could my relative slowness be the cause of my body position problems, or could it be that my bodyfat distribution has changed enough to mean I need to take corrective action? I have heard phrases like "pressing the T" and "downhill swimming", but don't really get them. Can somebody recommend a drill or drills that I could try? I feel that I understand the concepts of good body position, but can't come up with any techniques for improving it. Even if my problem really IS just how slow I am right now, there's always room for improving my body position. Sorry for such a long post on my first one, but I figured I'd better cover all the bases so I'd get more accurate advice.