Your legs will lose sensation or feeling from the swim to the bike. When you finally get your bike legs, it is often time to dismount, and start the run. Your first half mile on the run will fool you. Thinking you are hardly moving, you may be running quite fast. Then you finish, and it is a good feeling.
You may be out of breath, perhaps a little sweaty. But it feels great to have completed the tasks.
My advice for the beginner:
Know how to swim the distance required FIRST.
Get a good bike, and do at least three bike rides a week for at least three months leading up to your first triathlon.
Once a week, try to run within 4 minutes of dismounting the bike. This action is often called a brick by triathletes.
Have fun.
