Nice stuff Lars, clearly well suited to the moment.
Keep at it Jonesy, you can only benefit from persisting at it over time. It's great that you enjoyed your first time at it too, that's important. In a sense it's no different to practicing a repertoire piece, but it's simply a case of practicing the concept of "thinking ahead" whilst you're trying to compose on the spot. Conversely, there are other times when all you need do is play a chord and then think "okay, now where do I go?"
If extemporising at the end of a hymn, it's generally sensible to include snatches of the tune somewhere, but not necessarily mandatory. If you have the hymn tune in front of you, there is plenty of material to borrow and also plenty of "tricks" that can be applied. My favourites include minorising a major tune and vice-versa, playing it backwards (sometimes even with the written harmonies - that can have interesting results!!), or inverting it, or both. If it's a well written polyphonic arrangement, take one of the inner parts (like the tenor voice for example) and turn it into a soprano line. Or you can grab a short segment of the tune and turn it into an ostinato for some other part of the tune (or a new tune altogether). You can also grab an excerpt of the harmony and put something new over it. And if it's likely to go on for a while, modulation can provide additional interest. It's entirely up to you and the moment how you choose to change things - for that reason alone, it's why I find improvising/extemporising such fun!