A 2-year-old can be very easily carried on an adult's shoulders if you're in a huge hurry, but much of the time it's better to just slow down to the kid's speed. If kids get substantial practice walking every day, their speed and stamina improves quite quickly. By age ~3 they are quite able to keep up with a moderate adult pace, and by 4 they'll be running all around faster than you probably want to walk.