Millvale Riverfront Park is a GREAT place to start. The park is a tiny bit hard to find, so test drive it once first.
Basically, when I get head there I am heading NORTH on 28. I get off at Millvale. At the stop sign, (where Lincoln extends out ahead of me) I turn left (onto Ohio). At the stop sign (one short block) I turn left again (onto Grant). I pass under ONE bridge. The road ahead of me starts rising to where it meets with Route 28 near the 40th Street Bridge.
Before I get on any part of the road that is heading uphill, I am watching for a turn to the left, which will take me under a SECOND bridge. If you have good eyes, you will see a white and blue sign for Millvale Riverfont Park.
At the stop sign, turn right (only real option, and follow it to the big parking lot near Three Rivers Rowing Association. The trail starts (basically) on the sidewalk/path located between the building and the river.
While you are in the parking lot, you will see a sign telling you that you are at riverpost mile 2.8 -- or 2.8 miles from the Point. It's an easy/straight ride from there. This group can give you detailed directions to your final destination within downtown.
A word of caution! If you miss the entrance to the park coming from town,, you will end up on Route 28 heading north or Route 28 heading south.
I think the entrance to the park is easier to find if you are coming southbound on Route 28. From Mapquest, it looks like you would exit Rotue 28 and find yourself on Ohio Street. Continue straight as if you were heading for Route 28 South. Pass under one bridge, watch for small road, second bridge on your left.
If you pull up Millvale on Mapquest you can zoom in on the Route 28 Ramps. I did not look at the aerial, but my quess is that the entrance road to the park is about where the Exit 3 marker is on the Route 28 ramp configuration.
Sorry for the detailed directions. I get this inquiry a lot. I figured I might as well just get detailed instructions ready for the summer riding season.