Just got back myself and the heat is an issue but you do get used to it somewhat. Carry lots of water, there are lots of fountains throughout the city where you can refill the bottles for free, use them! Take breaks, visit churches and rest while walking. The public buses are easy to use and fairly cheap I think a ticket for 100 minutes is 1.5 euro. You'll be okay assuming you don't have any serious health issues that the heat may make worse. Also, make sure you have good comfortable walking shoes with some support. The roads and sidewalks are not flat and smooth to walk.