I walk 10,000 steps a day. Is that not enough?

It is excellent for your heart, your mood and your weight, and it is not strength training. The guidelines ask for both. Keep the steps and add two short strength sessions a week.

Will strength training slow me down or bulk me up?

Neither. Stronger legs make walking, cycling and swimming easier, because each stride or stroke uses a smaller share of what you have. Two sessions a week makes you stronger and firmer, not bigger.

How quickly will I notice?

Most people feel the difference in four to six weeks: the shopping in one trip, up off the floor without a hand, the heavy thing lifted. The early change is your nervous system learning to use the muscle you have.