What part of South Portland are you staying in? Most of the hotels in South Portland are in the Maine Mall / Airport area. It's about a 10 minute drive by car from there to the Old Port area, so I would say it's not walkable.
If you're staying in the residential area of South Portland across the river from Portland, then I guess you could walk, but it would be a bit of a hike. The Casco Bay bridge is fairly new with a pedestrian lane, but the bridge itself is almost a mile long and then you've got the distance to get to the bridge from wherever you're staying plus the distance from the bridge to the Old Port. if it were me I would drive.
We're going to be staying in the Marina, right next to the bridge... The bridge didn't look that long on the map, that's disappointing to hear... I guess we'll drive into Portland. Do you or does anyone have parking suggestions?
There are several parking garages in town. Rates don't differ too much. Metered street parking has a two hour limit during meter hours. You could always walk in and take a cab back if you don't feel like walking back. After crossing the bridge you'd have another half mile or so to reach the Old Port area, just to give you an idea of total distance. Or if you wanted a little more expensive but kind of fun option, the Casco Bay water taxi will take you one way from the Old Port to Spring Point Marina for $30.
I think walkable depends on how much you like to walk and how much of your day you mind giving over to walking for the exchange of not having to pay for transport. Walking across the bridge takes about 20 minutes (if you're walking about 3 mph) and I find it a pleasant enough walk. You step off the bridge at approximately 174 York Street on the Portland side. You'd be about 0.2 miles from the Victoria Mansion (uphill) or 0.2 miles from Becky's Diner (down hill).
Metered parking is free on Sundays and after 6 PM. The 2 hour limit is strict. (I honestly can't remember if the metered rate is 2 dollars an hour or 1 dollar an hour, I think it's 1). City owned garages are 2 dollars an hour. There are lots and garages owned by private companies. The privately owned lots are outrageously expensive, up to 5 dollars an hour. With rates like that you might find it worth it to walk in and cab back.
The length of the bridge won't be nearly as long from where you are staying. There is a park at the end of Waterman Drive, and there are stairs that go up ti the bridge from there.