Is 30 min/week of strength training worth it?

the2aces

A question for all those exercise physiologists out there...

I'm one of those people who has a really hard time fitting strength training into my routine. I'm in a great routine thus far, having lost 20lbs in the past 3 months mostly but counting calories, but certainly also by a significant addition of cardio. In the beginning I was having trouble keeping to 3-4 days per week of cardio, but for the past few weeks I've been averaging 4-5 days of 1 hour of cardio each, and have made great strides in my running abilities. I usually do a half hour of treadmill, and then a half hour of lower impact (on my knees) like elliptical/stationary bike combo.

I've always had a hard time replacing that second half hour with strength training because I'm caught up in the idea that it's not burning as many calories (you know - in vs. out). BUT, know that i do know all the benefits of strength training. I KNOW that it's important for injury prevention (been there done that), and I know that the muscle I turn my fat into will burn more calories at a resting rate. I know how good it is for me.

So, lately, after 3 days in a row of an hour of cardio at the gym, on the 4th day I've been getting a little...bored. So, after a half hour on the treadmill I actually WANT something different, and have been doing a 30-45 minute strength training circuit.

Short story long, here's my question. Am I doing anything by doing only one session of strength training per week, or is that pretty negligible?