You have named some of the the most common factors that women have to deal with when it comes to working out: 1. lack of time and 2. easy on the knees.
As you noted, elipitical training is a great workout that can really pack a heck of a caloric burn and it's nearly impact-free. However, like any form exercise, it can get monotonous. Also, you may need to increase your minutes of cardio since it sounds like you are already in good condition. The body does get stagnant quite easily, and a new challenge of a few extra minutes or a higher incline is just the thing to keep you on your toes, no pun intended.
If you can devote 45 minutes to cardio, it doesn't all have to be at one station. You can spend twenty on the recumbent bike on a "ride in the park" battling some grueling hills, and then jump on the eliptical for twenty-five minutes. That will give you spice of life, and still get your heart rate up and help you burn fat. Just be certain not to take too much time between machines if you want to stay in the proper heart rate zone.
Also, remember that every little bit of exercise counts. Take the stairs over the elevator, take that parking space off in the cut (as long as it's a safe, well-lit lot), etc... I do calf raises while I clean the kitchen and while I take showers. I also wear ankle weights around the house. Every little bit counts!
I highly recommend surfing ivillage.com for some great tips on squeezing fitness into a busy lifestyle. They also have great nutrition tips for women.
So basically, my answer is if you have a little block of time and you want to get in a cardio blast, the eliptical machine is a great way to go. But, I would also recommend that you occasionally try 45 minutes broken up. Here is my personal workout when I'm really short on time (upper body):
5 minutes on eliptical (working hard)
set of push ups (fast but very controlled)
25 crunches (control is key)
set of lat pull downs
5 minutes on eliptical (working hard)
set of push ups
25 crunches
set of tricep kickbacks (free weights; focusing on form)
5 minutes on eliptical (you should be sweating by now
)
set of push ups
25 of crunches
set of dumbbell presses (overhead; watch that form)
5 minutes on eliptical (really pushing yourself now, girlie)
25 of crunches
set of push ups
set of dumbbell curls
3 minutes on eliptical (pushing as hard as possible)
2 minutes cooling down on eliptical
*you should spend about five minutes doing the push ups, crunches, etc... between your time on the trainer. I cannot stress how key form is during this training. It's easy to get caught up in the pace of it and miss out on the benefits if you're not careful.
On days when you plan to focus on your lower body, you would keep the crunches, swap lunges for the push ups, and throw in sets of lower body exercises (i.e. leg curls, presses, calf raises). This would really give you the most benefit when you are pressed for time! After a workout like this, you still have time to shower and get that wig together!
Hope this helps! Talk with you soon!