Are you meeting half way or at her house?
If you do end up having to go, psyche yourself up in advance to tackle everything that she does that you have a problem with. If she ends up getting upset then you are better off no worse off if she buggers off and doesn't ever suggest meeting up again. As a minimum it should make the day a bit more bearable.
Every time she says something critical, pick her up on it, even if using the MN standard - did you mean to be so rude?
If she says that your choice of xxx is rubbish but yyy is better, then say no, I disagree. For me personally, I find that yyy doesn't work for me but xxx works really well. We're all different - there's no right or wrong answer.
That way you're taking the upper hand, not criticising her choice but validating your own (you could of course do the oh no, I find that one rubbish, this is much better if you want to be more assertive about things!)
If you go for a meal, and she starts talking about boring stuff, be over jolly and say oh come on, we're not old grannies yet, we're only meeting up for the day, let's not waste the time on boring stuff, let's talk about/do fun stuff/[insert whatever you want to do here]. And every time she switches onto boring stuff, just say no no no, we're not grannies today remember!!! and jolly her along.
Be brave and practise it all in your mind beforehand so you can do it and know what to say, so it sounds natural and not nasty or odd.
Alternatively, look at the national rail website and see if there are any problems on the bit of rail that you are needing to travel on - any planned works where your journey might be thrown onto buses or delayed? Or something cropping up at home that needs dealing with? or a weather report that means it's better not to travel? Or just be non-specific and say that something has cropped up so you're going to have to cancel this time. And don't get drawn on the details.
good luck!