Well, I've dabbled with audaxes and I quite like 100k rides. I've tried a few 200, but they are too long.
I've done 2 imperial centuries this year, and I was quite pleased with how I got on, but my experience is that the step up to longer distances just inflicts too much pain.
Do you have regular riding companions or do you tend to go it alone?
If you really want to do audaxes, there are permanents and DIY stuff. I've done one permanent (the Manningtree 150) but I've not bothered with any DIY. If I want to do a ride, I'll work out a route and do it at my own pace. I can't be arsed with sticking to a lower time limit.
I always think the best rides are the ones you plan yourself, or pals do, and that take you to several pubs. We had a great one a couple of weeks ago with the Grand Union Canal towpath ride.
But Morpeth is a fair old trek from Essex... trains stop there though, don't they?