Lausanne is built on steep hills, and walking everywhere will test your legs. The city compensates with an excellent metro system — the M2 line runs from the lakefront at Ouchy up through the centre to the northern suburbs, and is the steepest metro in the world. Buses fill in the gaps.
If you are staying overnight, your hotel should provide a free Lausanne Transport Card covering all public transport for the duration of your stay. The funicular from the Flon to the old town is useful for saving your knees. The lakefront promenade from Ouchy is flat and pleasant for walking.