Walking Fiesole to Piazzale Michelangelo, Florence

8 km (5 miles) or so, under 2 hours to something over 2 hours depending on how you walk and how often you stop.

Down quite a lot, then flat, then over the river and up quite a lot.

Because Florence is in a bowl, surrounded on all sides, heat tends to come early and stay. I find Florence hotter feeling than Rome, even though the actual temperature in Rome may be a little higher.

Florence is as hot as hell in July, so it is better to start early in order to be back before noon, when heat may make too much walking uncomfortable - or maybe in very very late afternoon as Fiesole is fresher than Florence at night. Personally, I would walk down through the narrow Via della Piazzola, that is shadier and a more pleasant walk than the standard car itinerary. Not too much to see but narrow alleys down the hill are nice walks.
Once you are down at viale Volta or piazzale delle Cure you may even take a bus back as the last section back to the center is not really interesting. You can check yourself with google Street view.

Please note that buses to Fiesole now leave from S. Marco area. I do not understand your reference to piazzale Michelangelo.

lachera,

have a look at https://en.wikipedia.org/wiki/Piazzale_Michelangelo

If you do the walk in the evening, make sure to have bug spray/repellent. That was the only place I can recall in Italy (and this was mid-Sept) where I got bothered by mosquitoes...we didn't do the walk, but took the bus to Fiesole as the sun was setting to see the view - the mosquitoes drove us away.

Per a previous comment, walking down the Via della Piazzuola would make for a much more pleasant stroll than the more direct route down Via S. Domenico (the bus route). Once down the hill it gets pretty congested and walking it on a hot day wouldn't be very pleasant. Catching a taxi or bus across town to Piazzale Michelangelo for your photo op would save time, aggravation and your legs on a sultry day.
You don't say whether you're staying in Fiesole, but if you'll be spending any time exploring the pretty little town I'd suggest an hour or two exploring the Roman ruins, which are quite nice. And for a slightly off-the-beaten-path excursion you could hike out to Monte Ceceri where there's a memorial to Leonardo's "first flight" attempt in 1505...400 years before Kitty Hawk.

Nigel, it is only 29 years I live in Florence so maybe I understand what piazzale Michelangelo is. What I do not understand, the original poster asks in the title to walk back to piazzale Michelangelo, while in the text she ask about walking back to the river, that of course is not piazzale Michelangelo.

Thank you all for your replies.
Looks like we'll take the bus up early morning and, depending on the weather, walk down via della Piazzuola.
I see that Piazzale Michelangelo is not close to the river after all.
Ciao, Lynne