I had a neighbor like that, only we lived in separate apts, but in the same house. and once the weather got warm, she'd bring her lawn chair out, and sit in the sun right outside her door, on the driveway. Where I had to pass to get to MY car or back into my apt.. Like you, in the beginning I would stop and chat, but sometimes I'd be there a good 30 minutes before I could even get into my apt. And even when I'd say, ok, need to go in now, she'd follow me across the driveway, still talking. It got to the point where if I had to go out, i lurk to hear her go inside, and then run out to my car, so I could avoid her.
she too is nice, but needy, and would start talking about something, then wander off onto some tangent and start discussing people I didn't know, and so on. what I finally started doing was the "walk and talk" I'd say hello, how are you, but not stop moving, and eventually I think she got the message that I wasn't available for prolonged chats anymore.
So I'd just continue doing what you do, and maybe wave and yell hello, but then go back to your stuff. don't feel guilty about not going over to chat; her lonliness isn't your issue