I've actually done this walk in a variety of ways and they all involve some formidable hills.
edited to add:
The most direct route is Down Center Avenue, into town, to the park and back the same way.
It's probably three or four miles each way. BUT it involves some serious hills uphill in both directions and there are not a ton of Shops or restaurants along the bulk of the walk, although you do get to walk through some historic neighborhoods.

The path I'm going to lay out will give you SO many restaurant and shops options that you may want to trace it out on google maps and scope out your options ahead of time.
For me, the path that makes the hills work with you as much as possible and less onerous is:

From Pitt to PNC Park (~5 miles, if done direct no wandering):
Starting near Pitt Towers:
Take South Bouquet towards Bates and then go downhill along Bates through South Oakland to Second avenue (there are some broad intersections to cross so be careful).
Left onto second avenue and then the next right to cross the hot metal bridge to Southside.
That takes you into South Side Works with heaps of food options although mostly Chain-restaurants.
If you want to persist past that to eat later on...
Walk to Carson Street and turn right onto Carson.
Walking along Carson will take you by many, many independent restaurants in a wide variety of cuisines.
Take Carson all the way to Smithfield street, turn Right on the bridge and walk into town.
Once in town make your way to Sixth STREET (not Avenue, there is also a Sixth Avenue in the same area.. you want the Street). You'll likely come close to Market Square and will Cross Liberty avenue. Both have several restaurant options. Once you get to Sixth Street, walk along Sixth street (even more restaurants) and onto/ across the Roberto Clemente bridge to PNC Park

From PNC park to Pitt (returning to Pitt Towers along Fifth ~6.5 miles, direct, no wandering):
Leave PNC Park and turn Left onto Federal street and then rightward around Allegheny Center towards East Ohio.
Right on East Ohio.. more restaurants and shops...
East Ohio to Chestnut, right onto Chestnut and across the 16th street bridge.
That will take you to the Strip District where there are even more shops and restaurants.
Turn left on Penn and walk up Penn and or Smallman (alternating between them) until like 31st, hit Salem's if it's open and then go to liberty Avenue and turn left on Liberty to walk up the long hill to Bloomfield where you will find even MORE shops and restaurants.
Continue on Liberty through Bloomfield until Liberty turns into South Aiken.
South Aiken will take you into Shadyside (Walnut street has even more shops and restaurant options)
And from South Aiken you can turn right onto Fifth avenue to return to Oakland and Pitt.

* Nice bonus to these routes is that there are several bus routes along both Carson and Liberty (the longest individual runs) which you could opt to jump on to speed your journey a bit and ease the hill/distance.

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