So, you'll probably get other folks advise here along with mine. I wanted to share about the weight gain and muscle work. Also suggest a strategy for those last 10 pounds.
First, muscle weighs more than fat. As you build muscle, you could be gaining muscle weight but losing fat. What is your final goal: firm, trim body that healthy and strong for long and productive life? You might find the weight gain fine in that case. For me, I needed to lose straight weight to relieve my knees from pain. So, I'm carefully losing weight to a specific target while also watching out for firmness and being trim (which is my ultimate goal but my short-term is just weight).
Also, bone is heavy. This is built up along with the muscles. It is good to have strong bones.
My suggestion, and watch for others' advise here, is to consider HIIT (or high intensity interval training). You can google this and check out the various blogs within Prevention magazine and books along with 3fatchicks and other places. I've been very happy with my Firm 500 calorie burn workout dvd which is basically along similar lines. So, I've been looking up on-line more about HIIT. It really does seem extremely easy. So, I've been researching a bit more to make sure I'm not missing anything.
Basically, it's a two-phase cycle of x seconds of intense high exercise cycled with x or y seconds of lower level exercise. So, I've been doing 30 seconds of jumping jacks, 30 seconds gentle jogging in place, 30 seconds "mountain climber", 30 seconds gentle jogging in place. I have a long list of kinds of high intensity activities for my living room: jumping jacks, high knees, jumping tires, football drills, in-line skater, butt kicks, "burpies", etc.
I've built up a lot of indurance so I do these intervals for 30-45 minutes depending on my schedule for the morning (also with a 55 or 65 minute aerobics exercise dvd).
I have a six pound goal to lose before vacation in three weeks. So, I'm very closely tracking my calories and doing intervals (which I've just started in last couple days).