Something to remember about jogging strollers - if you actually plan on jogging or running with it get the fixed front wheel, NOT the one that swivels. The swiveling one is great for trails and walking, but there's a pretty good risk of overturning the stroller if you're jogging with it. The fixed front wheel is a little harder to turn with, but you get used to it quickly.

Depending on the size of your baby, strapping him into a baby carrier (one that fits on your back or your front) might be an option for walks. I did that with my middle child, and he really enjoyed our little walks. Most of the carriers I've seen were resonably inexpensive.

I am in the same position as you, I have a 3 mo old as well! Also a 3.5 yr old to complicate things. I would be careful about using a jogging stroller yet--their bobble heads are not up for the strain of the jiggling and jostling if you are running with it. I walk with the baby in a carrier but it is not enough cardio for me. When I try to put in a dvd the baby never sleeps long enough, and I have to cut my workout short, before I even break a sweat. Soooo frustrating!!! I feel like I need to sweat everyday, but it is just not happening. I feel fat and stuck in the same place. I wish I had a treadmill or elliptical... I'm going to try Leslie Sansome's walking dvd and see if I can make it 3 miles--the pace is pretty good and I can put the baby in a wrap and do it.