@pinkpowerofthought
YANBU. I hate 'poppers in' and people who think you should drop anything and everything you have planned, to accommodate them. It's selfish and narcissistic. No, it's not all about you. 
Some people I know think they can just pop in at 2 minutes notice - (like they just text us and say they will be there soon, or even worse, they just turn up!) They must think me and DH are just sitting there waiting for them to call us and grace us with their presence. Fucks me right off it does. I mean, what if we were shagging, or on our way out, or are ill, or have other people round, or are about to have a cozy afternoon watching a film coz we hadn't seen each other much for a week?!
I would be contacting the friend if I were you and saying you cannot make it. She sounds a bit entitled to me. I don't care how long she has been a friend, or how far away she lives; a decent friend would not be asking someone to meet up, and drop everything to see her, at 24 hours notice. It's all very well people saying 'just meet her for an hour,' but it won't be an hour will it? You need to get ready, (get dressed/put make up on etc,) then there is the travelling time, and parking and getting to the place you're meeting, then she will keep you longer for an hour... Then you need to get back. The whole process could take at LEAST 4-5 hours. Including getting ready and getting there and back.
Even if you arrange to meet at 11.30am for an 'hour,' you will need to start getting ready at 9.30-10am, and you need to allow time to get there and park up etc etc, then you won't get back til 2pm probably. So the whole day is affected really.
I know someone who lives 20 minutes walk away from me, and she has texted before and said 'me and hubby are coming for a walk around in a minute; we thought we'd pop in and see you and your hubby.' Yeah coz we are just sitting here waiting for people to 'pop in,' and we have fuckall else to do. 
When I text back and say 'no, not right now, coz we are busy/on our way out etc, maybe another time...' she never responds. She can't bear to be told 'no' so goes into a sulk and refuses to respond. Any normal person would text back and say 'ok, never mind, maybe another time,' not just ignore the person they had messaged!
Makes me wonder how people manage to keep friends when they do this. My normal friends, arrange something a week or two in advance with me. And we meet at the pub or a cafe. We don't invade each other's homes at 5 minutes notice, and expect each other to drop anything we're doing, and entertain us for 3 or 4 hours.
Horrible, narcissistic, entitled behaviour.