The Arc de Triomphe is huge. You can reach it from the Champs Élysées at the intersection with the Grand Palais. If you think it's just a short walk, think again. It probably takes a solid hour to get there.
The walk up the Champs Élysées is indeed pleasant but I'd recommend doing it first thing in the morning, especially in summer. You access Arc de Triomphe via a tunnel that goes under the road and leaves you in the square. This is the only way in.