We are heading there for 3 days at the end of February. I signed up for a personal greeter tour:

http://www.marseilleprovencegreeters.com/en/

We indicated what our interests are and the guide will tailor the walk to our likes.

It is never a bad idea to spend a few hours in Marseille if you have the opportunity. The Vieux Port and the view from Notre Dame de la Garde are often considered to be the main sights, but this year Marseille is the European capital of culture and there are all sorts of new things to see, including brand new museums.

You are likely to be very frustrated to have so little time there.

I would also guess the area around the Vieux Port is the logical place to hang out for the most part, as well as going up to see Notre Dame de la Garde. YOu need to do a little advance research to find out how to do that, but there are buses that go up there that leave from very close to the port. If you have time, you could also walk around the old quarter which is just to the north of the port, there are some interesting buildings and museums in that area. http://www.marseille-provence.info/marseille-marseilles/what-to-see-tourist-attractions/45-marseille-the-old-town-panier.html

You can also ride the tourist trains that go from the old port to Notre Dame de la Garde.

It's uphill all the way, so at least you get to see things in comfort. You can get off at the top and take another train back down.

We toured the highligts of the city in 5-6 hours so your plan is doable based on real experience.
We bougt a city pass which includes tourist trains,boat to the If Castle(Chateau d'If),all public transport tickets...etc.
Here is the link:
http://www.marseille-tourisme.com/en/travel-trade/tourist-offers/city-pass/
As Rastaguytoday mentioned above,the tourist train is the best option to go to Cathedral Notre Dame de la Garde.

Thanks to everyone for responding to my question about Marseille - way back in January! I apologize for the delay. (We had a small crisis here and I forgot) We leave for Marseille in one week and I'm hoping that DebitNM can let us know how it went with the Global Greeters in that city. I'm wondering if this organization truly consists of "volunteers" who love and want to share their city with visitors, or just another scheme to get your Euros. Thanks!

We absolutely loved our free tour. And yes it was free, although we did give a donation.

Our tour guide, Partick, was amazing. He is a retired gentleman who was born in Marseille and had such a deep love of his city. He spoke excellent English and is a traveler himself, so it was easy do spend time with him.

While the tour was supposed to be 2 hours, he actually spent the entire day with us! While we paid for his lunch and a drink afterward, he paid our bus fare up to Notre Dame; bought us a bag of Navettes at http://www.fourdesnavettes.com/ and took us see such a diverse range of sites within Marseille.

I can't think of a better way to see Marseille.

I am planning on a day trip to Marseille this summer (from Aix). Do you think it's possible to combine with Cassis or would that be pushing it?

If you are wanting to drive the Corniche des Crêtes, possible boat ride in the calanques and see Marseille, I would say that is really too much to see and do in a single day.

I think the people who does those "free" tours are just looking for another way to make money, they make it off tips. It is expected to tip them, so it isn't really they are doing it just to be nice.