My neighbour keeps trying to invite me in for a cup of tea

My partner and I moved in at the same time as next door moved into their house. When they first moved in, I felt the lady in particular was trying too hard to be my best friend right away. I don't work like that. It takes me ages to decide if someone is a friend and that's because I want them to be not because it's forced upon me. She talks A LOT and is hard to get away from. She also is a know it all which i find incredibly difficult. Anyway,  18 months later we all get on (not bezzie mates though), and I'm really grateful to have GOOD respectful neighbours as I know you can't choose them. She's at home all day with her baby. I understand she gets bored but that's not my fault. It's just, she often asks me in for a brew if i see her. I've done it a few times to keep things ticking over, not cos I wanted to,  but end up staying far too long. It's like I'm trapped. 

I went round the other day to get a parcel after work. I was shattered, it was my first week back in work for almost 3 months. She even said how tired I looked. She asked me several times to stay for tea but I politely declined. I was still there over half an hour. She said several times near the end "we should have had that cup of tea after all. I'll ask you next time and you'll say yes and you'll stay even if you only drink half". (She is foreign so it wasn't as forceful as it sounds, it's how she uses English language). It doesn't matter what I say I can't get away. 

There's just no concept of bending to other people's needs or saying "hello, here's your parcel, goodbye". How do I manage these situations?? I'm a very passive person and slowly learning to be more assertive but when I'm caught off guard or really tired and have less brain power I don't know how to manage it.