I was kinda in the same boat. When my daughter turned about 3, after a day of trying to keep up at the playground, was like WOW, she's getting faster and stronger and I'm getting older and slower. The next day when I woke in pain, I knew it was time to start doin something. That has snowballed into a bit of an obsesion and, aside from having to buy a new wardrobe, feel it was 1 of the best changes I ever made.
1spd has some great ideas, and certainly put some good advice out there but would like to add that, a total body work out routine is great for a newbie, or someone who hasn't done that type routine in a while, but a split routine will allow you to focus all your energy available for that day on a few given body parts. So lets say,
Monday you do back. When you do back your also doing biceps because when you PULL the weight to exercise your back, you use your biceps, so concentrate on those 2 groups for that day.
Wednesday you do chest. When you PUSH the weight to exercise chest, you also do shoulders and triceps so concentrate on that group that day. ect.
So on Wednesday you can use a different set of muscles that are not sore or still recovering from Monday.
I think the push, pull concept is a sound 1 but more concentated, as in, when doing push on chest with bench press, then do pull on chest with flys in that same work out, on a given day.
Core, calf, and forearm muscles, recuperate, and plateau faster, so it's a good idea to do a different form of core exercise with each and every work out.
Also think it's worth mentioning that the biggest muscle group is your legs and will naturaly boost your testosterone allowing you to push yourself harder, and should be the basis of any routine. IMO you should have a day just for legs and do some at the begining of each work out. i.e. some form of squat.
Push yoursef a little harder with either more reps, more curcuits, or more weight, or less rest in between each exercise each week.
Everyone has an off day, but if you feel like your not getting stronger, it's either you hit a plateau, so change routine, or your over training and should take a week off.
Change your routine every 6-8 weeks at the most to help avoid plateaus.
For building endorence, while doing any routine or cardio, try to get your heart rate as high as you can to be able to keep it there for 45-60 minutes. To go longer once in a while is cool but in general anything longer than an hour when it comes to building muscle is counter productive.
Always warm up before, and stretch after to maximize efforts and avoid injury.
For anyone looking for a great example of a split routine, check out Lee Hayward's website and look for his 12 week program. It's free.