If you're a curious person like me and want to get a better handle on the numbers, I suggest investing in a good heart rate monitor. I have had the Polar FT60 for a little over a year and I love it! It also allows you to put in your info (height/weight/age) and then has you do a resting heart rate test and gives you your estimated fitness level. Then you can choose a program (fitness improving, weight loss, etc) and it will tell you how many minutes to shoot for each week in different heart rate zones. That helped me realize that I was working out far too much in "Zone 3, high intensity" and not enough in Zone 2, moderate intensity. Just another fun gadget to play with and help you stay on track!
Someone who is busier than you is working out right now.
There will come a day when you can no longer do this. Today is not that day.