Hi, we are fit (ish) and walk often. It took us about 20 minutes to get to the bridge - could be done in 15 mins but crowds and activities meant we took our time. Crossing the bridge is again a crowd management activity as much as a walk - took only 5 minutes to cross. You are then on the main river-front tourist strip, which we tended to avoid. We either walked up the 200+ steep steps to get to Porto center or through the river-front throng and then up towards 'town', which offered less steps but was still a 'climb'. Depends exactly where you want to go. But about 40 minutes walk is a good planning number - certainly not more than that. If walking is not your thing, we found the various ride shares and taxis to be affordable and reliable. The main strip on the Gaia side of the river from Rebello to the bridge has lots of bar and eating options - perhaps as much as the opposite bank of the river? Again, it is touristy but fun nonetheless.