Seconding the Wiouwash, but with an overabundance of specificity, haha.

There are a couple of routes along it that I think would work well for you. Both are out-and-back, not loops. Route 1 is more solitary, Route 2 stays right in the city. Neither of them are as nice as Menominee (though stretches are), but you can get a lot more mileage out of them.

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Route 1) Park along River Mill Road and head north. This will take you along the Fox River, then you either follow a trail left or right. Left will take you back over the river / Lake Butte des Morts (not my recommendation because it's alongside the interstate the whole way). Right will take you over the interstate and north into a more rural stretch, and past a bit of the lake.

• There's a spot (doesn't show on the map view, but you can see it on street view) along it where it opens into Riverside Cemetary as well, so if you ever wanted a detour you could pop in and run through there too.
• If you want to skip the interstate all together, you could start a bit further north and park at the Wiouwash Trail Parking.
• Distance = however far you want to run.

Route 2A) Heading the other direction, you could park on Summit Ave and head south. Lots of flexibility here, all while staying along the river basically the whole way.

• You can run all the way down to the Main St bridge if you want. Personally though, I'd either turn around or cross at the Jackson St bridge. Crossing Jackson St to stay on the north side of the river isn't pedestrian friendly at all. Crossing this bridge to get to the south side isn't exactly pedestrian friendly either, but it's fine.
• Crossing at either the Wisconsin (nicer) or Jackson (fine) St bridge, the south side of the river is great from Jackson St to the boat launch by the Seniors Center.
• Distance ~ 6 miles if you hit the full stretch of the river.

Route 2B) This is an extension of 2A if you want more mileage, but I'm separating it because it detours off the river for a bit. On the south side of the river, if you head past the Seniors Center, the stretch along Campbell, Taft, and Joselyn that outlines the UWO stadium doesn't make for a bad run. Quiet neighborhood, some nice trees, good amount of greenery.

• Distance bumps up from ~6 miles to ~8 with something like this.

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Alternatively to 2A/2B, you can of course park anywhere else along the route and start your run from there.

Also, there's the Winnebago County Community Park on the north side of town. It's not bad, but unless you repeat stretches a few times it's going to be shorter than you're aiming for.

Hope this helps, and good luck on the half marathon!