Have you seen the map on wikitravel for San Juan? it may be the same as you have seen online but it does show a few points of interest and also agrees with Lisa about wearing comfortable shoes. Last time I was in San Juan it was pouring with rain in the early evening which put a dampener on wandering around but hopefully it will be ok when you arrive.
Just want to warn you about a scammer who targets tourists & cruise day trippers in old San Juan. He starts off by asking if you speak English and then sometimes appears to be helpful by telling you about safe areas or starts to tell you how him and his father got mugged and his father is in hospital. He'll say police are unhelpful and he needs about $7 to get to the hospital to get to his father. Stop him before he goes into his story by saying you've read about him on the internet.
He's been doing this scam for a few years so don't get conned by his hard luck story if he approaches you. With luck you won't even meet him.