How do I convince a skinny middle-aged guy like myself that it's OK to gain weight / muscle?

I'm a man in his 50s who grew up a fat kid but slimmed down during adulthood thanks to 25 years of marathon running, which kept me skinny but had to stop five years ago due to a bad knee.

During those five years I've been doing some pretty intense daily cardio on the stairmaster for, seriously, two hours a morning, which helps me burn 1,400 calories a day, because that fat-kid inside me is terrified of gaining weight. My problem is that I've noticed that as I get older I have to increase my daily cardio time just to maintain my weight.

While I like being skinny I realize I have to refocus on improving strength and flexibility. I have very little muscle mass, and I know I have to do something to change that even though it means seeing my weight creep up (which pains me because I put so much of my self worth into the number on the scale).

This all makes me wonder:

-- Does my current strategy of two hours of daily cardio excessive? Would so much cardio negatively affecting my health?

-- How do I replace calorie burn from cardio into strength training? How much daily strength training do I need to do to stay in shape?

-- Can anyone else share their experiences with being middle aged and starting strength training for the first time?