The simple way would be:
Jail Trail to Grant Street, then take the shared use sidewalk to Smithfield. Then Smithfield to Liberty, left on 10th, right on Penn, left on Smallman, and follow Smallman the rest of the way 'til you can cut up to Penn.
Yes, there will be some traffic, but it shouldn't be too bad. There are probably some other, quieter options that others can suggest, too.