Exercise advice, please

Hello. I need some advice/opinions, but first a little background (I'll try to keep it concise).
I used to be an avid exerciser/gym rat. I always believed, and would preach to others, "there is always time"....Well, that was in my single, childless days. Well, over the last 4.5 years I have had two children, gone from working part-time to full time, and my husband is just now getting back to work after being laid off for 15 months. I've had to give up my gym membership for many reasons. Over the past 4.5 years my exercise has gone to nil, and I've picked up horrible eating habits and now I'm about 20 lbs heavier than I want or need to be. So, I've decided its time for a change. Or rather, a return to a former self. In keeping with the attitude that "there is always time", I've committed to waking up at 5:00 am every morning before the kids wake up and its time to get ready for work and daycare. At 5:00am I walk for 30 mins on my treadmill. Most days, I walk for another 20-25 minutes at my lunch break as well (weather permitting). This is mon-fri. On the 6th day (Sat or Sun) I walk for 60 minutes on my treadmill or outdoors (but most times I have one of the kids with me so treadmill is easier). I'm logging my food daily. MFP has me at 1220 calories per day, and I always eat back my exercise calories (usually between 100-190 calories). It has been 20 days and I've lost 5.5 lbs and I am very pleased with that.
Here is my question, I KNOW I need to add strength training. My dilemma is, if I replace one of my morning walk sessions with strength training, I'm afraid the loss of calories burned will affect my weight loss. Especially because my lunchtime walk at work isn't always at the toughest pace. I do have to return to work so I can't get too sweaty, and sometimes I have a co-worker with me and it slows me down (but I enjoy the social aspect).
Some side notes, I can not do high impact activities...I have a knee issue that I am trying to avoid surgery for. Also, I can not exercise at night. My kids are in bed by 8:30, but I need that time before I go to bed to study for a certification exam that will advance my career, spend some time with my husband who now works long hours, and also to prepare lunches for the next day, etc. (wow, that sounds like a lot of excuses...but all true).
Final question, is resistance tubing just as good as hand held weights? Anybody successful with those? I like the idea of buying one piece of equipment and being able to vary resistance.
Sorry so long. If you've gotten this far, thanks for reading:)