Once upon a time, when in Rome, we walked from our apartment (we were already staying at the gate, Porta Metronia). We walked the Appian Way and took our chances on the Catacombs, which we managed to tour without huge crowds. There was a restaurant on the Appian Way, where we enjoyed a nice lunch. As many years have passed, I don't have the information handy.
So, I think it's a pleasant thing to do on a lovely day.
If you have the patience for a guided 4-6 hour tour (that seems long to me), then go for it. If you don't, I think with a bit of research, you can put together an independent tour. Start by looking up the tickets/times for the Catacombs and any self-guided itineraries.