Mark,
I feel your pain on the age/metabolism slow down issue..I just hit 30, and it happened to me in a big way....actually, it started about 5-6 years ago with me, but eating tex-mex food and drinking beer all the time didn't help either. I've toned down my diet a lot, especially during the week, and increased my lifting and cardio quite a bit, and I've lost about 17 pounds in a little over a year....not too bad, especially since I'm not all that disciplined with my diet, and I don't follow a precise cardio/lifting split.
One thing you might consider is doing some light riding, or jogging on your leg day...keep it in zone 2 (60-70% MHR), so that you'll still be in that fat burning zone, but you won't be killing yourself for your later workout. Then, crank up the intensity in a big way on your cardio days. Not sure what kinda shape your joints are in for running, but I guarantee you, if you cross-train and change things up a little and throw in some running, it'll cut down on the boredom factor, and you'll be able to get your heart rate up in a hurry....It's also a little easier to control your intervals...course, you need a heart rate monitor to verify this, but you can get some good ones for cheap on the net these days....great tool.
Good luck with the workouts bro....like I said, if you find that stuff..just zap it over to me...